Nicholas LeFevre, MD, MSAM, FAAFP

Ruptured abdominal aortic aneurysms (AAAs) caused about 6000 deaths annually in the United States between 2014 and 2020¹ and are associated with a pooled mortality rate of 81%.² They result from a distinct degenerative process of the layers of the aortic wall.² An AAA is defined as an abdominal aorta whose dilation is > 50% normal (more commonly, a diameter > 3 cm).^3,4 The risk for rupture correlates closely with size; most ruptures occur in aneurysms > 5.5 cm^3,4 (TABLE 1⁵).

Most AAAs are asymptomatic and often go undetected until rupture, resulting in poor outcomes. Because of a low and declining prevalence of AAA and ruptured AAA in developed countries, screening recommendations target high-risk groups rather than the general population.^4,6-8 This review summarizes risk factors, prevalence, and current evidence-based screening and management recommendations for AAA.

Who’s at risk?

Age is the most significant nonmodifiable risk factor, with AAA rupture uncommon in patients younger than 55 years.⁹ One retrospective study found the odds ratio (OR) for diagnosing AAA was 9.41 in adults ages 65 to 69 years (95% CI, 8.76-10.12; P < .0001) and 14.46 (95% CI, 13.45-15.55; P < .0001) in adults ages 70 to 74 years, compared to adults younger than 55 years.¹⁰

Smoking is the most potent modifiable risk factor for AAA. Among patients with AAA, > 90% have a history of smoking.⁴ The association between smoking and AAA is dose dependent, with an OR of 2.61 (95% CI, 2.47-2.74) in patients with a pack-per-year history < 5 years and 12.13 (95% CI, 11.66-12.61) in patients with a pack-per-year history > 35 years, compared to nonsmokers.¹⁰ The risk for AAA increases with smoking duration but decreases with cessation duration.^4,10 Smoking cessation remains an important intervention, as active smokers have higher AAA rupture rates.¹¹

Other risk factors for AAA include concomitant cardiovascular disease (CVD) such as coronary artery disease (CAD), cerebrovascular disease, atherosclerosis, dyslipidemia, and hypertension.¹⁰ Factors associated with reduced risk for AAA include African American race, Hispanic ethnicity, Asian ethnicity, diabetes, smoking cessation, consuming fruits and vegetables > 3 times per week, and exercising more than once per week.^6,10

Prevalence declines but sex-based disparities in outcomes persist

The prevalence of AAA has declined in the United States and Europe in recent decades, correlating with declining rates of smoking.^4,12 Reports published between 2011 and 2019 estimate that AAA prevalence in men older than 60 years has declined over time, with a prevalence of 1.2% to 3.3%.⁶ The prevalence of AAA has also decreased in women,^6,13,14 estimated in 1 study to be as low as 0.74%.¹³ Similarly, deaths from ruptured AAA have declined markedly in the United States—by 70% between 1999 and 2016 according to 1 analysis.⁹

One striking difference in the male-female data is that although AAAs are more common in men, there is a 2- to 4-fold higher risk for rupture in women, who account for nearly half of all AAA-related deaths.^9,10,15-17 The reasons for this heightened risk to women despite lower prevalence are not fully understood but are likely multifactorial and related to a general lack of screening for AAA in women, tendency for AAA to rupture at smaller diameters in women, rupture at an older age in women, and a history of worse surgical outcomes in women than men (though the gap in surgical outcomes appears to be closing).^9,10,18

Continue to: While declines in AAA and AAA-related...

While declines in AAA and AAA-related death are largely attributed to lower smoking rates, other likely contributing factors include the implementation of screening programs, incidental detection during cross-sectional imaging, and improved surgical techniques and management of CV risk factors (eg, hypertension, hyperlipidemia).^9,10

The benefits of screening older men

Randomized controlled trials (RCTs) have demonstrated the benefits of AAA screening programs. A meta-analysis of 4 population­based RCTs of AAA screening in men ≥ 65 years demonstrated statistically significant reductions in AAA rupture (OR = 0.62; 95% CI, 0.55-0.70) and death from AAA (OR = 0.65; 95% CI, 0.57-0.74) over 12 to 15 years, with a number needed to screen (NNS) of 305 (95% CI, 248-411) to prevent 1 AAA-related death.¹⁸ The study also found screening decreases the rate of emergent surgeries for AAA (OR = 0.57; 95% CI, 0.48-0.68) while increasing the number of elective surgeries (OR = 1.44; 95% CI, 1.34-1.55) over 4 to 15 years.¹⁸

Only 1 study has demonstrated an improvement in all-cause mortality with screening programs, with a relatively small benefit (OR = 0.97; 95% CI, 0.94-0.99).¹⁹ Only 1 of the studies included women and, while underpowered, showed no difference in AAA-related death or rupture.²⁰ Guidelines and recommendations of various countries and professional societies focus screening on subgroups at highest risk for AAA.^4,6-8,18

Screening recommendations from USPSTF and others

The US Preventive Services Task Force ­(USPSTF) currently recommends one-time ultrasound screening for AAA in men ages 65 to 75 years who have ever smoked (commonly defined as having smoked > 100 cigarettes) in their lifetime.⁶ This grade “B” recommendation, initially made in 2005 and reaffirmed in the 2014 and 2019 ­USPSTF updates, recommends screening the ­highest-risk segment of the population (ie, older male smokers).⁶

In men ages 65 to 75 years with no smoking history, rather than routine screening, the USPSTF recommends selectively offering screening based on the patient’s medical history, family history, risk factors, and personal values (with a “C” grade).⁶ The USPSTF continues to recommend against screening for AAA in women with no smoking history and no family history of AAA.⁶ According to the USPSTF, the evidence is insufficient to recommend for or against screening women ages 65 to 75 years who have ever smoked or have a family history of AAA (“I” statement).⁶

Continue to: One critique of the USPSTF recommendations

One critique of the USPSTF recommendations is that they fail to detect a significant portion of patients with AAA and AAA rupture. For example, in a retrospective analysis of 55,197 patients undergoing AAA repair, only 33% would have been detected by the USPSTF grade “B” recommendation to screen male smokers ages 65 to 75 years, and an analysis of AAA-related fatalities found 43% would be missed by USPSTF criteria.^9,21

Screening guidelines from the Society for Vascular Surgery (SVS) are broader than those of the USPSTF, in an attempt to capture a larger percentage of the population at risk for AAA-related disease by extrapolating from epidemiologic data. The SVS guidelines include screening for women ages 65 to 75 years with a smoking history, screening men and women ages 65 to 75 years who have a first-degree relative with AAA, and consideration of screening patients older than 75 years if they are in good health and have a first-degree relative with AAA or a smoking history and have not been previously screened.⁴ However, these expanded recommendations are not supported by patient-oriented evidence.⁶

Attempts to broaden screening guidelines must be tempered by potential risks for harm, primarily overdiagnosis (ie, diagnosing AAAs that would not otherwise rise to clinical significance) and overtreatment (ie, resulting in unnecessary imaging, appointments, anxiety, or surgery). Negative psychological effects on quality of life after a diagnosis of AAA have not been shown to cause significant harm.^6,18

A recent UK analysis found that screening programs for AAA in women modeled after those in men are not cost effective, with an NNS to prevent 1 death of 3900 in women vs 700 in men.^15,18 Another recent trial of ultrasound screening in 5200 high-risk women ages 65 to 74 years found an AAA incidence of 0.29% (95% CI, 0.18%-0.48%) in which only 3 large aneurysms were identified.²²

Smoking is the most potent modifiable risk factor for abdominal aortic aneurysm.

In the United States, rates of screening for AAA remain low.²³ One study has shown electronic medical record–based reminders increased screening rates from 48% to 80%.²⁴ Point-of-care bedside ultrasound performed by clinicians also could improve screening rates. Multiple studies have demonstrated that screening and diagnosis of AAA can be performed safely and effectively at the bedside by nonradiologists such as family physicians and emergency physicians.^25-28 In 1 study, such exams added < 4 minutes to the patient encounter.²⁶ Follow-up surveillance schedules for those identified as having a AAA are summarized in TABLE 2.⁴

Continue to: Management options

Management options: Immediate repair or surveillance?

After diagnosing AAA, important decisions must be made regarding management, including indications for surgical repair, appropriate follow-up surveillance, and medications for secondary prevention and cardiovascular risk reduction.

EVAR vs open repair

The 2 main surgical strategies for aneurysm repair are open repair and endovascular repair (EVAR). In the United States, EVAR is becoming the more common approach and was used to repair asymptomatic aneurysms in > 80% of patients and ruptured aneurysms in 50% of patients.⁶ There have been multiple RCTs assessing EVAR and open repair for large and small aneurysms.^29-34 Findings across these studies consistently show EVAR is associated with lower immediate (ie, ­30-day) morbidity and mortality but no ­longer-term survival benefit compared to open repair.

EVAR procedures require ongoing long-term surveillance for endovascular leakage and other complications, resulting in an increased need for re-intervention.^31,33,35 For these reasons, the National Institute for Health and Care Excellence (NICE) guidelines suggest open repair as the preferred modality.⁷ However, SVS and the American College of Cardiology Foundation/American Heart Association guidance support either EVAR or open repair, noting that open repair may be preferable in patients unable to engage in long-term follow-up surveillance.³⁶

Indications for repair. In general, repair is indicated when an aneurysm reaches or exceeds 5.5 cm.^4,7 Both SVS and NICE also recommend clinicians consider surgical repair of smaller, rapidly expanding aneurysms (> 1 cm over a 1-year period).^4,7 Based on evidence suggesting a higher risk for rupture in women with smaller aneurysms,^14,37 SVS recommends clinicians consider surgical repair in women with an AAA ≥ 5.0 cm. Several RCTs evaluating the benefits of immediate repair for smaller-sized aneurysms (4.0-5.5 cm) favored surveillance.^38,39 Accepted indications for surgical repair are summarized in TABLE 3.^4,7,34Surgical repair recommendations also are based on aneurysm morphology, which can be fusiform or saccular (FIGURE). More than 90% of AAAs are fusiform.⁴⁰ Although saccular AAAs are less common, some studies suggest they are more prone to rupture than fusiform AAAs, and SVS guidelines suggest surgical repair of saccular aneurysms regardless of size.^4,41,42

Perioperative and long-term risks. Both EVAR and open repair of AAA carry a high perioperative and long-term risk for death, as patients often have multiple comorbidities. A 2019 trial comparing EVAR to open repair with 14 years of follow-up reported death in 68% of patients in the EVAR group and 70% in the open repair group. ³¹ Among these deaths, 2.7% in the EVAR group and 3.7% in the open repair group were aneurysm related.³¹ The study also found a second surgical intervention was required in 19.8% of patients in the open repair group and 26.7% in the EVAR group.³¹

Continue to: When assessing perioperative risk...

Although abdominal aortic aneurysms are more common in men, there is a 2- to 4-fold higher risk for rupture in women.

When assessing perioperative risk, SVS guidelines recommend clinicians employ a shared decision-making approach with patients that incorporates Vascular Quality Initiative (VQI) mortality risk score.⁴ (VQI risk calculators are available at https://qxmd.com/vascular-study-group-new-england-decision-support-tools.⁴³)

Medication management

Based on the close association of aortic aneurysm with atherosclerotic CVD (ASCVD), professional societies such as the European Society of Cardiology and European Atherosclerosis Society (ESC/EAS) have suggested aortic aneurysm is equivalent to ASCVD and should be managed medically in a similar manner to peripheral arterial disease.⁴⁴ Indeed, many patients with AAA may have concomitant CAD or other arterial vascular diseases (eg, carotid, lower extremity).

Statins. In its guidelines, the ESC/EAS consider patients with AAA at “very high risk” for adverse CV events and suggest pharmacotherapy with high-intensity statins, adding ezetimibe or proprotein convertase ­subtilisin/kexin type 9 (PCSK9) inhibitors if needed, to reduce low-density lipoprotein cholesterol ≥ 50% from baseline, with a goal of < 55 mg/dL.⁴⁴ Statin therapy additionally lowers all-cause postoperative mortality in patients undergoing AAA repair but does not affect the rate of aneurysm expansion.⁴⁵

Aspirin and other anticoagulants. Although aspirin therapy may be indicated for the secondary prevention of other cardiovascular events that may coexist with AAA, it does not appear to affect the rate of growth or prevent rupture of aneurysms.^46,47 In addition to aspirin, anticoagulants such as clopidogrel, enoxaparin, and warfarin are not recommended when the presence of AAA is the only indication.⁴

The USPSTF continues to recommend against screening in women with no smoking history and no family history of abdominal aortic aneurysm.

Other medications. Angiotensin-­converting enzyme inhibitors, angiotensin receptor blockers, beta-blockers, and antibiotics (eg, doxycycline) have been studied as a treatment for AAA. However, none has shown benefit in reducing aneurysm growth or rupture and they are not recommended for that sole purpose.^4,48

Metformin. There is a negative association between diabetes and AAA expansion and rupture. Several cohort studies have indicated that this may be an independent effect driven primarily by exposure to metformin. While it is not unreasonable to consider this another important indication for metformin use in patients with diabetes, RCT evidence has yet to establish a role for metformin in patients without diabetes who have AAA.^48,49

ACKNOWLEDGEMENT
The authors thank Gwen Wilson, MLS, AHIP, for her assistance with the literature searches performed in the preparation of this manuscript.

CORRESPONDENCE
Nicholas LeFevre, MD, Family and Community Medicine, University of Missouri–Columbia School of Medicine, One Hospital Drive, M224 Medical Science Building, Columbia, MO 65212; [email protected]

Ruptured abdominal aortic aneurysms (AAAs) caused about 6000 deaths annually in the United States between 2014 and 2020¹ and are associated with a pooled mortality rate of 81%.² They result from a distinct degenerative process of the layers of the aortic wall.² An AAA is defined as an abdominal aorta whose dilation is > 50% normal (more commonly, a diameter > 3 cm).^3,4 The risk for rupture correlates closely with size; most ruptures occur in aneurysms > 5.5 cm^3,4 (TABLE 1⁵).

Most AAAs are asymptomatic and often go undetected until rupture, resulting in poor outcomes. Because of a low and declining prevalence of AAA and ruptured AAA in developed countries, screening recommendations target high-risk groups rather than the general population.^4,6-8 This review summarizes risk factors, prevalence, and current evidence-based screening and management recommendations for AAA.

Who’s at risk?

Age is the most significant nonmodifiable risk factor, with AAA rupture uncommon in patients younger than 55 years.⁹ One retrospective study found the odds ratio (OR) for diagnosing AAA was 9.41 in adults ages 65 to 69 years (95% CI, 8.76-10.12; P < .0001) and 14.46 (95% CI, 13.45-15.55; P < .0001) in adults ages 70 to 74 years, compared to adults younger than 55 years.¹⁰

Smoking is the most potent modifiable risk factor for AAA. Among patients with AAA, > 90% have a history of smoking.⁴ The association between smoking and AAA is dose dependent, with an OR of 2.61 (95% CI, 2.47-2.74) in patients with a pack-per-year history < 5 years and 12.13 (95% CI, 11.66-12.61) in patients with a pack-per-year history > 35 years, compared to nonsmokers.¹⁰ The risk for AAA increases with smoking duration but decreases with cessation duration.^4,10 Smoking cessation remains an important intervention, as active smokers have higher AAA rupture rates.¹¹

Other risk factors for AAA include concomitant cardiovascular disease (CVD) such as coronary artery disease (CAD), cerebrovascular disease, atherosclerosis, dyslipidemia, and hypertension.¹⁰ Factors associated with reduced risk for AAA include African American race, Hispanic ethnicity, Asian ethnicity, diabetes, smoking cessation, consuming fruits and vegetables > 3 times per week, and exercising more than once per week.^6,10

Prevalence declines but sex-based disparities in outcomes persist

The prevalence of AAA has declined in the United States and Europe in recent decades, correlating with declining rates of smoking.^4,12 Reports published between 2011 and 2019 estimate that AAA prevalence in men older than 60 years has declined over time, with a prevalence of 1.2% to 3.3%.⁶ The prevalence of AAA has also decreased in women,^6,13,14 estimated in 1 study to be as low as 0.74%.¹³ Similarly, deaths from ruptured AAA have declined markedly in the United States—by 70% between 1999 and 2016 according to 1 analysis.⁹

One striking difference in the male-female data is that although AAAs are more common in men, there is a 2- to 4-fold higher risk for rupture in women, who account for nearly half of all AAA-related deaths.^9,10,15-17 The reasons for this heightened risk to women despite lower prevalence are not fully understood but are likely multifactorial and related to a general lack of screening for AAA in women, tendency for AAA to rupture at smaller diameters in women, rupture at an older age in women, and a history of worse surgical outcomes in women than men (though the gap in surgical outcomes appears to be closing).^9,10,18

Continue to: While declines in AAA and AAA-related...

While declines in AAA and AAA-related death are largely attributed to lower smoking rates, other likely contributing factors include the implementation of screening programs, incidental detection during cross-sectional imaging, and improved surgical techniques and management of CV risk factors (eg, hypertension, hyperlipidemia).^9,10

The benefits of screening older men

Randomized controlled trials (RCTs) have demonstrated the benefits of AAA screening programs. A meta-analysis of 4 population­based RCTs of AAA screening in men ≥ 65 years demonstrated statistically significant reductions in AAA rupture (OR = 0.62; 95% CI, 0.55-0.70) and death from AAA (OR = 0.65; 95% CI, 0.57-0.74) over 12 to 15 years, with a number needed to screen (NNS) of 305 (95% CI, 248-411) to prevent 1 AAA-related death.¹⁸ The study also found screening decreases the rate of emergent surgeries for AAA (OR = 0.57; 95% CI, 0.48-0.68) while increasing the number of elective surgeries (OR = 1.44; 95% CI, 1.34-1.55) over 4 to 15 years.¹⁸

Only 1 study has demonstrated an improvement in all-cause mortality with screening programs, with a relatively small benefit (OR = 0.97; 95% CI, 0.94-0.99).¹⁹ Only 1 of the studies included women and, while underpowered, showed no difference in AAA-related death or rupture.²⁰ Guidelines and recommendations of various countries and professional societies focus screening on subgroups at highest risk for AAA.^4,6-8,18

Screening recommendations from USPSTF and others

The US Preventive Services Task Force ­(USPSTF) currently recommends one-time ultrasound screening for AAA in men ages 65 to 75 years who have ever smoked (commonly defined as having smoked > 100 cigarettes) in their lifetime.⁶ This grade “B” recommendation, initially made in 2005 and reaffirmed in the 2014 and 2019 ­USPSTF updates, recommends screening the ­highest-risk segment of the population (ie, older male smokers).⁶

In men ages 65 to 75 years with no smoking history, rather than routine screening, the USPSTF recommends selectively offering screening based on the patient’s medical history, family history, risk factors, and personal values (with a “C” grade).⁶ The USPSTF continues to recommend against screening for AAA in women with no smoking history and no family history of AAA.⁶ According to the USPSTF, the evidence is insufficient to recommend for or against screening women ages 65 to 75 years who have ever smoked or have a family history of AAA (“I” statement).⁶

Continue to: One critique of the USPSTF recommendations

One critique of the USPSTF recommendations is that they fail to detect a significant portion of patients with AAA and AAA rupture. For example, in a retrospective analysis of 55,197 patients undergoing AAA repair, only 33% would have been detected by the USPSTF grade “B” recommendation to screen male smokers ages 65 to 75 years, and an analysis of AAA-related fatalities found 43% would be missed by USPSTF criteria.^9,21

Screening guidelines from the Society for Vascular Surgery (SVS) are broader than those of the USPSTF, in an attempt to capture a larger percentage of the population at risk for AAA-related disease by extrapolating from epidemiologic data. The SVS guidelines include screening for women ages 65 to 75 years with a smoking history, screening men and women ages 65 to 75 years who have a first-degree relative with AAA, and consideration of screening patients older than 75 years if they are in good health and have a first-degree relative with AAA or a smoking history and have not been previously screened.⁴ However, these expanded recommendations are not supported by patient-oriented evidence.⁶

Attempts to broaden screening guidelines must be tempered by potential risks for harm, primarily overdiagnosis (ie, diagnosing AAAs that would not otherwise rise to clinical significance) and overtreatment (ie, resulting in unnecessary imaging, appointments, anxiety, or surgery). Negative psychological effects on quality of life after a diagnosis of AAA have not been shown to cause significant harm.^6,18

A recent UK analysis found that screening programs for AAA in women modeled after those in men are not cost effective, with an NNS to prevent 1 death of 3900 in women vs 700 in men.^15,18 Another recent trial of ultrasound screening in 5200 high-risk women ages 65 to 74 years found an AAA incidence of 0.29% (95% CI, 0.18%-0.48%) in which only 3 large aneurysms were identified.²²

Smoking is the most potent modifiable risk factor for abdominal aortic aneurysm.

In the United States, rates of screening for AAA remain low.²³ One study has shown electronic medical record–based reminders increased screening rates from 48% to 80%.²⁴ Point-of-care bedside ultrasound performed by clinicians also could improve screening rates. Multiple studies have demonstrated that screening and diagnosis of AAA can be performed safely and effectively at the bedside by nonradiologists such as family physicians and emergency physicians.^25-28 In 1 study, such exams added < 4 minutes to the patient encounter.²⁶ Follow-up surveillance schedules for those identified as having a AAA are summarized in TABLE 2.⁴

Continue to: Management options

Management options: Immediate repair or surveillance?

After diagnosing AAA, important decisions must be made regarding management, including indications for surgical repair, appropriate follow-up surveillance, and medications for secondary prevention and cardiovascular risk reduction.

EVAR vs open repair

The 2 main surgical strategies for aneurysm repair are open repair and endovascular repair (EVAR). In the United States, EVAR is becoming the more common approach and was used to repair asymptomatic aneurysms in > 80% of patients and ruptured aneurysms in 50% of patients.⁶ There have been multiple RCTs assessing EVAR and open repair for large and small aneurysms.^29-34 Findings across these studies consistently show EVAR is associated with lower immediate (ie, ­30-day) morbidity and mortality but no ­longer-term survival benefit compared to open repair.

EVAR procedures require ongoing long-term surveillance for endovascular leakage and other complications, resulting in an increased need for re-intervention.^31,33,35 For these reasons, the National Institute for Health and Care Excellence (NICE) guidelines suggest open repair as the preferred modality.⁷ However, SVS and the American College of Cardiology Foundation/American Heart Association guidance support either EVAR or open repair, noting that open repair may be preferable in patients unable to engage in long-term follow-up surveillance.³⁶

Indications for repair. In general, repair is indicated when an aneurysm reaches or exceeds 5.5 cm.^4,7 Both SVS and NICE also recommend clinicians consider surgical repair of smaller, rapidly expanding aneurysms (> 1 cm over a 1-year period).^4,7 Based on evidence suggesting a higher risk for rupture in women with smaller aneurysms,^14,37 SVS recommends clinicians consider surgical repair in women with an AAA ≥ 5.0 cm. Several RCTs evaluating the benefits of immediate repair for smaller-sized aneurysms (4.0-5.5 cm) favored surveillance.^38,39 Accepted indications for surgical repair are summarized in TABLE 3.^4,7,34Surgical repair recommendations also are based on aneurysm morphology, which can be fusiform or saccular (FIGURE). More than 90% of AAAs are fusiform.⁴⁰ Although saccular AAAs are less common, some studies suggest they are more prone to rupture than fusiform AAAs, and SVS guidelines suggest surgical repair of saccular aneurysms regardless of size.^4,41,42

Perioperative and long-term risks. Both EVAR and open repair of AAA carry a high perioperative and long-term risk for death, as patients often have multiple comorbidities. A 2019 trial comparing EVAR to open repair with 14 years of follow-up reported death in 68% of patients in the EVAR group and 70% in the open repair group. ³¹ Among these deaths, 2.7% in the EVAR group and 3.7% in the open repair group were aneurysm related.³¹ The study also found a second surgical intervention was required in 19.8% of patients in the open repair group and 26.7% in the EVAR group.³¹

Continue to: When assessing perioperative risk...

Although abdominal aortic aneurysms are more common in men, there is a 2- to 4-fold higher risk for rupture in women.

When assessing perioperative risk, SVS guidelines recommend clinicians employ a shared decision-making approach with patients that incorporates Vascular Quality Initiative (VQI) mortality risk score.⁴ (VQI risk calculators are available at https://qxmd.com/vascular-study-group-new-england-decision-support-tools.⁴³)

Medication management

Based on the close association of aortic aneurysm with atherosclerotic CVD (ASCVD), professional societies such as the European Society of Cardiology and European Atherosclerosis Society (ESC/EAS) have suggested aortic aneurysm is equivalent to ASCVD and should be managed medically in a similar manner to peripheral arterial disease.⁴⁴ Indeed, many patients with AAA may have concomitant CAD or other arterial vascular diseases (eg, carotid, lower extremity).

Statins. In its guidelines, the ESC/EAS consider patients with AAA at “very high risk” for adverse CV events and suggest pharmacotherapy with high-intensity statins, adding ezetimibe or proprotein convertase ­subtilisin/kexin type 9 (PCSK9) inhibitors if needed, to reduce low-density lipoprotein cholesterol ≥ 50% from baseline, with a goal of < 55 mg/dL.⁴⁴ Statin therapy additionally lowers all-cause postoperative mortality in patients undergoing AAA repair but does not affect the rate of aneurysm expansion.⁴⁵

Aspirin and other anticoagulants. Although aspirin therapy may be indicated for the secondary prevention of other cardiovascular events that may coexist with AAA, it does not appear to affect the rate of growth or prevent rupture of aneurysms.^46,47 In addition to aspirin, anticoagulants such as clopidogrel, enoxaparin, and warfarin are not recommended when the presence of AAA is the only indication.⁴

The USPSTF continues to recommend against screening in women with no smoking history and no family history of abdominal aortic aneurysm.

Other medications. Angiotensin-­converting enzyme inhibitors, angiotensin receptor blockers, beta-blockers, and antibiotics (eg, doxycycline) have been studied as a treatment for AAA. However, none has shown benefit in reducing aneurysm growth or rupture and they are not recommended for that sole purpose.^4,48

Metformin. There is a negative association between diabetes and AAA expansion and rupture. Several cohort studies have indicated that this may be an independent effect driven primarily by exposure to metformin. While it is not unreasonable to consider this another important indication for metformin use in patients with diabetes, RCT evidence has yet to establish a role for metformin in patients without diabetes who have AAA.^48,49

ACKNOWLEDGEMENT
The authors thank Gwen Wilson, MLS, AHIP, for her assistance with the literature searches performed in the preparation of this manuscript.

CORRESPONDENCE
Nicholas LeFevre, MD, Family and Community Medicine, University of Missouri–Columbia School of Medicine, One Hospital Drive, M224 Medical Science Building, Columbia, MO 65212; [email protected]

Ruptured abdominal aortic aneurysms (AAAs) caused about 6000 deaths annually in the United States between 2014 and 2020¹ and are associated with a pooled mortality rate of 81%.² They result from a distinct degenerative process of the layers of the aortic wall.² An AAA is defined as an abdominal aorta whose dilation is > 50% normal (more commonly, a diameter > 3 cm).^3,4 The risk for rupture correlates closely with size; most ruptures occur in aneurysms > 5.5 cm^3,4 (TABLE 1⁵).

Most AAAs are asymptomatic and often go undetected until rupture, resulting in poor outcomes. Because of a low and declining prevalence of AAA and ruptured AAA in developed countries, screening recommendations target high-risk groups rather than the general population.^4,6-8 This review summarizes risk factors, prevalence, and current evidence-based screening and management recommendations for AAA.

Who’s at risk?

Age is the most significant nonmodifiable risk factor, with AAA rupture uncommon in patients younger than 55 years.⁹ One retrospective study found the odds ratio (OR) for diagnosing AAA was 9.41 in adults ages 65 to 69 years (95% CI, 8.76-10.12; P < .0001) and 14.46 (95% CI, 13.45-15.55; P < .0001) in adults ages 70 to 74 years, compared to adults younger than 55 years.¹⁰

Smoking is the most potent modifiable risk factor for AAA. Among patients with AAA, > 90% have a history of smoking.⁴ The association between smoking and AAA is dose dependent, with an OR of 2.61 (95% CI, 2.47-2.74) in patients with a pack-per-year history < 5 years and 12.13 (95% CI, 11.66-12.61) in patients with a pack-per-year history > 35 years, compared to nonsmokers.¹⁰ The risk for AAA increases with smoking duration but decreases with cessation duration.^4,10 Smoking cessation remains an important intervention, as active smokers have higher AAA rupture rates.¹¹

Other risk factors for AAA include concomitant cardiovascular disease (CVD) such as coronary artery disease (CAD), cerebrovascular disease, atherosclerosis, dyslipidemia, and hypertension.¹⁰ Factors associated with reduced risk for AAA include African American race, Hispanic ethnicity, Asian ethnicity, diabetes, smoking cessation, consuming fruits and vegetables > 3 times per week, and exercising more than once per week.^6,10

Prevalence declines but sex-based disparities in outcomes persist

The prevalence of AAA has declined in the United States and Europe in recent decades, correlating with declining rates of smoking.^4,12 Reports published between 2011 and 2019 estimate that AAA prevalence in men older than 60 years has declined over time, with a prevalence of 1.2% to 3.3%.⁶ The prevalence of AAA has also decreased in women,^6,13,14 estimated in 1 study to be as low as 0.74%.¹³ Similarly, deaths from ruptured AAA have declined markedly in the United States—by 70% between 1999 and 2016 according to 1 analysis.⁹

One striking difference in the male-female data is that although AAAs are more common in men, there is a 2- to 4-fold higher risk for rupture in women, who account for nearly half of all AAA-related deaths.^9,10,15-17 The reasons for this heightened risk to women despite lower prevalence are not fully understood but are likely multifactorial and related to a general lack of screening for AAA in women, tendency for AAA to rupture at smaller diameters in women, rupture at an older age in women, and a history of worse surgical outcomes in women than men (though the gap in surgical outcomes appears to be closing).^9,10,18

Continue to: While declines in AAA and AAA-related...

While declines in AAA and AAA-related death are largely attributed to lower smoking rates, other likely contributing factors include the implementation of screening programs, incidental detection during cross-sectional imaging, and improved surgical techniques and management of CV risk factors (eg, hypertension, hyperlipidemia).^9,10

The benefits of screening older men

Randomized controlled trials (RCTs) have demonstrated the benefits of AAA screening programs. A meta-analysis of 4 population­based RCTs of AAA screening in men ≥ 65 years demonstrated statistically significant reductions in AAA rupture (OR = 0.62; 95% CI, 0.55-0.70) and death from AAA (OR = 0.65; 95% CI, 0.57-0.74) over 12 to 15 years, with a number needed to screen (NNS) of 305 (95% CI, 248-411) to prevent 1 AAA-related death.¹⁸ The study also found screening decreases the rate of emergent surgeries for AAA (OR = 0.57; 95% CI, 0.48-0.68) while increasing the number of elective surgeries (OR = 1.44; 95% CI, 1.34-1.55) over 4 to 15 years.¹⁸

Only 1 study has demonstrated an improvement in all-cause mortality with screening programs, with a relatively small benefit (OR = 0.97; 95% CI, 0.94-0.99).¹⁹ Only 1 of the studies included women and, while underpowered, showed no difference in AAA-related death or rupture.²⁰ Guidelines and recommendations of various countries and professional societies focus screening on subgroups at highest risk for AAA.^4,6-8,18

Screening recommendations from USPSTF and others

The US Preventive Services Task Force ­(USPSTF) currently recommends one-time ultrasound screening for AAA in men ages 65 to 75 years who have ever smoked (commonly defined as having smoked > 100 cigarettes) in their lifetime.⁶ This grade “B” recommendation, initially made in 2005 and reaffirmed in the 2014 and 2019 ­USPSTF updates, recommends screening the ­highest-risk segment of the population (ie, older male smokers).⁶

In men ages 65 to 75 years with no smoking history, rather than routine screening, the USPSTF recommends selectively offering screening based on the patient’s medical history, family history, risk factors, and personal values (with a “C” grade).⁶ The USPSTF continues to recommend against screening for AAA in women with no smoking history and no family history of AAA.⁶ According to the USPSTF, the evidence is insufficient to recommend for or against screening women ages 65 to 75 years who have ever smoked or have a family history of AAA (“I” statement).⁶

Continue to: One critique of the USPSTF recommendations

One critique of the USPSTF recommendations is that they fail to detect a significant portion of patients with AAA and AAA rupture. For example, in a retrospective analysis of 55,197 patients undergoing AAA repair, only 33% would have been detected by the USPSTF grade “B” recommendation to screen male smokers ages 65 to 75 years, and an analysis of AAA-related fatalities found 43% would be missed by USPSTF criteria.^9,21

Screening guidelines from the Society for Vascular Surgery (SVS) are broader than those of the USPSTF, in an attempt to capture a larger percentage of the population at risk for AAA-related disease by extrapolating from epidemiologic data. The SVS guidelines include screening for women ages 65 to 75 years with a smoking history, screening men and women ages 65 to 75 years who have a first-degree relative with AAA, and consideration of screening patients older than 75 years if they are in good health and have a first-degree relative with AAA or a smoking history and have not been previously screened.⁴ However, these expanded recommendations are not supported by patient-oriented evidence.⁶

Attempts to broaden screening guidelines must be tempered by potential risks for harm, primarily overdiagnosis (ie, diagnosing AAAs that would not otherwise rise to clinical significance) and overtreatment (ie, resulting in unnecessary imaging, appointments, anxiety, or surgery). Negative psychological effects on quality of life after a diagnosis of AAA have not been shown to cause significant harm.^6,18

A recent UK analysis found that screening programs for AAA in women modeled after those in men are not cost effective, with an NNS to prevent 1 death of 3900 in women vs 700 in men.^15,18 Another recent trial of ultrasound screening in 5200 high-risk women ages 65 to 74 years found an AAA incidence of 0.29% (95% CI, 0.18%-0.48%) in which only 3 large aneurysms were identified.²²

Smoking is the most potent modifiable risk factor for abdominal aortic aneurysm.

In the United States, rates of screening for AAA remain low.²³ One study has shown electronic medical record–based reminders increased screening rates from 48% to 80%.²⁴ Point-of-care bedside ultrasound performed by clinicians also could improve screening rates. Multiple studies have demonstrated that screening and diagnosis of AAA can be performed safely and effectively at the bedside by nonradiologists such as family physicians and emergency physicians.^25-28 In 1 study, such exams added < 4 minutes to the patient encounter.²⁶ Follow-up surveillance schedules for those identified as having a AAA are summarized in TABLE 2.⁴

Continue to: Management options

Management options: Immediate repair or surveillance?

After diagnosing AAA, important decisions must be made regarding management, including indications for surgical repair, appropriate follow-up surveillance, and medications for secondary prevention and cardiovascular risk reduction.

EVAR vs open repair

The 2 main surgical strategies for aneurysm repair are open repair and endovascular repair (EVAR). In the United States, EVAR is becoming the more common approach and was used to repair asymptomatic aneurysms in > 80% of patients and ruptured aneurysms in 50% of patients.⁶ There have been multiple RCTs assessing EVAR and open repair for large and small aneurysms.^29-34 Findings across these studies consistently show EVAR is associated with lower immediate (ie, ­30-day) morbidity and mortality but no ­longer-term survival benefit compared to open repair.

EVAR procedures require ongoing long-term surveillance for endovascular leakage and other complications, resulting in an increased need for re-intervention.^31,33,35 For these reasons, the National Institute for Health and Care Excellence (NICE) guidelines suggest open repair as the preferred modality.⁷ However, SVS and the American College of Cardiology Foundation/American Heart Association guidance support either EVAR or open repair, noting that open repair may be preferable in patients unable to engage in long-term follow-up surveillance.³⁶

Indications for repair. In general, repair is indicated when an aneurysm reaches or exceeds 5.5 cm.^4,7 Both SVS and NICE also recommend clinicians consider surgical repair of smaller, rapidly expanding aneurysms (> 1 cm over a 1-year period).^4,7 Based on evidence suggesting a higher risk for rupture in women with smaller aneurysms,^14,37 SVS recommends clinicians consider surgical repair in women with an AAA ≥ 5.0 cm. Several RCTs evaluating the benefits of immediate repair for smaller-sized aneurysms (4.0-5.5 cm) favored surveillance.^38,39 Accepted indications for surgical repair are summarized in TABLE 3.^4,7,34Surgical repair recommendations also are based on aneurysm morphology, which can be fusiform or saccular (FIGURE). More than 90% of AAAs are fusiform.⁴⁰ Although saccular AAAs are less common, some studies suggest they are more prone to rupture than fusiform AAAs, and SVS guidelines suggest surgical repair of saccular aneurysms regardless of size.^4,41,42

Perioperative and long-term risks. Both EVAR and open repair of AAA carry a high perioperative and long-term risk for death, as patients often have multiple comorbidities. A 2019 trial comparing EVAR to open repair with 14 years of follow-up reported death in 68% of patients in the EVAR group and 70% in the open repair group. ³¹ Among these deaths, 2.7% in the EVAR group and 3.7% in the open repair group were aneurysm related.³¹ The study also found a second surgical intervention was required in 19.8% of patients in the open repair group and 26.7% in the EVAR group.³¹

Continue to: When assessing perioperative risk...

Although abdominal aortic aneurysms are more common in men, there is a 2- to 4-fold higher risk for rupture in women.

When assessing perioperative risk, SVS guidelines recommend clinicians employ a shared decision-making approach with patients that incorporates Vascular Quality Initiative (VQI) mortality risk score.⁴ (VQI risk calculators are available at https://qxmd.com/vascular-study-group-new-england-decision-support-tools.⁴³)

Medication management

Based on the close association of aortic aneurysm with atherosclerotic CVD (ASCVD), professional societies such as the European Society of Cardiology and European Atherosclerosis Society (ESC/EAS) have suggested aortic aneurysm is equivalent to ASCVD and should be managed medically in a similar manner to peripheral arterial disease.⁴⁴ Indeed, many patients with AAA may have concomitant CAD or other arterial vascular diseases (eg, carotid, lower extremity).

Statins. In its guidelines, the ESC/EAS consider patients with AAA at “very high risk” for adverse CV events and suggest pharmacotherapy with high-intensity statins, adding ezetimibe or proprotein convertase ­subtilisin/kexin type 9 (PCSK9) inhibitors if needed, to reduce low-density lipoprotein cholesterol ≥ 50% from baseline, with a goal of < 55 mg/dL.⁴⁴ Statin therapy additionally lowers all-cause postoperative mortality in patients undergoing AAA repair but does not affect the rate of aneurysm expansion.⁴⁵

Aspirin and other anticoagulants. Although aspirin therapy may be indicated for the secondary prevention of other cardiovascular events that may coexist with AAA, it does not appear to affect the rate of growth or prevent rupture of aneurysms.^46,47 In addition to aspirin, anticoagulants such as clopidogrel, enoxaparin, and warfarin are not recommended when the presence of AAA is the only indication.⁴

The USPSTF continues to recommend against screening in women with no smoking history and no family history of abdominal aortic aneurysm.

Other medications. Angiotensin-­converting enzyme inhibitors, angiotensin receptor blockers, beta-blockers, and antibiotics (eg, doxycycline) have been studied as a treatment for AAA. However, none has shown benefit in reducing aneurysm growth or rupture and they are not recommended for that sole purpose.^4,48

Metformin. There is a negative association between diabetes and AAA expansion and rupture. Several cohort studies have indicated that this may be an independent effect driven primarily by exposure to metformin. While it is not unreasonable to consider this another important indication for metformin use in patients with diabetes, RCT evidence has yet to establish a role for metformin in patients without diabetes who have AAA.^48,49

ACKNOWLEDGEMENT
The authors thank Gwen Wilson, MLS, AHIP, for her assistance with the literature searches performed in the preparation of this manuscript.

CORRESPONDENCE
Nicholas LeFevre, MD, Family and Community Medicine, University of Missouri–Columbia School of Medicine, One Hospital Drive, M224 Medical Science Building, Columbia, MO 65212; [email protected]

Subclinical hypothyroidism (SCH) is a biochemical state in which the thyroid-stimulating hormone (TSH) is elevated while the free thyroxine (T4) level is normal. Overt hypothyroidism is not diagnosed until the free T4 level is decreased, regardless of the degree of TSH elevation.

The overall prevalence of SCH in iodine-rich areas is 4% to 10%, with a risk for progression to overt hypothyroidism of between 2% and 6% annually.¹ The prevalence of SCH varies depending on the TSH reference range used.¹ The normal reference range for TSH varies depending on the laboratory and/or the reference population surveyed, with the range likely widening with increasing age.

SCH is most common among women, the elderly, and White individuals.² The discovery of SCH is often incidental, given that usually it is detected by laboratory findings alone without associated symptoms of overt hypothyroidism.³

The not-so-significant role of symptoms in subclinical hypothyroidism

Symptoms associated with overt hypothyroidism include constipation, dry skin, fatigue, slow thinking, poor memory, muscle cramps, weakness, and cold intolerance. In SCH, these symptoms are inconsistent, with around 1 in 3 patients having no symptoms at all.⁴

One study reported that roughly 18% of euthyroid individuals, 22% of SCH patients, and 26% of those with overt hypothyroidism reported 4 or more symptoms classically thought to be related to hypothyroidism.⁴ A large Danish cohort study found that hypothyroid symptoms were no more common in patients with SCH than in euthyroid individuals in the general population.⁵ These studies question the validity of attributing symptoms to SCH.

Adverse health associations

Observational data suggest that SCH is associated with an increased risk for dyslipidemia, coronary heart disease, heart failure, and cardiovascular mortality, particularly in those with TSH levels ≥ 10 mIU/L.^6,7 Such associations were not found for most adults with TSH levels between 5 and 10 mIU/L.⁸ There are also potential associations of SCH with obesity, nonalcoholic fatty liver disease, and nonalcoholic steatohepatitis.^9,10 Despite thyroid studies being commonly ordered as part of a mental health evaluation, SCH has not been statistically associated with depressive symptoms.^11,12

Caveats with laboratory testing

There are several issues to consider when performing a laboratory assessment of thyroid function. TSH levels fluctuate considerably during the day, as TSH secretion has a circadian rhythm. TSH values are 50% higher at night and in the early morning than during the rest of the day.¹³ TSH values also may rise in response to current illness or stress. Due to this biologic variability, repeat testing to confirm TSH levels is recommended if an initial test result is abnormal.¹⁴

Continue to: An exact reference range...

Supplements containing biotin should be withheld for several days before assessing thyroid function.

An exact reference range for TSH is not widely agreed upon—although most laboratories regard 4.0 to 5.0 mIU/L as the high-end cutoff for normal. Also, “normal” TSH levels appear to differ by age. Accordingly, some experts have recommended an age-based reference range for TSH levels,¹⁵ although this is not implemented widely by laboratories. A TSH level of 6.0 mIU/L (or even higher) may be more appropriate for adults older than 65 years.¹

Biotin supplementation has been shown to cause spurious thyroid testing results (TSH, T3, T4) depending on the type of assay used. Therefore, supplements containing biotin should be withheld for several days before assessing thyroid function.¹⁶Patients with SCH are often categorized as having TSH levels between 4.5 and 10 mIU/L (around 90% of patients) or levels ≥ 10 mIU/L.^8,17 If followed for 5 years, approximately 60% of patients with SCH and TSH levels between 4 and 10 mIU/L will normalize without intervention.¹⁸ Normalization is less common in patients with a TSH level greater than 10 mIU/L.¹⁸

The risk for progression to overt hypothyroidism also appears to be higher for those with certain risk factors. These include higher baseline TSH levels, presence of thyroid peroxidase antibodies (TPOAbs), or history of neck irradiation or radioactive iodine uptake.¹ Other risk factors for eventual thyroid dysfunction include female sex, older age, goiter, and high iodine intake.¹³

Evidence for treatment varies

Guidelines for the treatment of SCH (TABLE 1^8,14,19,20) are founded on the condition’s risk for progression to overt hypothyroidism and its association with health consequences such as cardiovascular disease. Guidelines of the American Thyroid Association (ATA) and European Thyroid Association (ETA), and those of the United Kingdom–based National Institute for Health and Care Excellence (NICE), prioritize treatment for individuals with a TSH level > 10 mIU/L^a and for those with TSH values < 10 mIU/L but still elevated and apparent symptoms of hypothyroidism.^14,19,20 The strength of evidence behind this guidance is challenged by a lack of data from prospective randomized controlled trials (RCTs) demonstrating health benefits following treatment of SCH. The British Medical Journal (BMJ) Guideline cites this lack of evidence and recommends against treating SCH at any TSH level, regardless of symptoms.⁸

There are few large RCTs of treatment outcomes for SCH. A 2017 RCT (the Thyroid Hormone Replacement for Untreated Older Adults with Subclinical Hypothyroidism, or TRUST, trial) of 737 adults older than 65 years with SCH evaluated the ability of levothyroxine to normalize TSH values compared with placebo. At 1 year, there was no difference in hypothyroid symptoms or tiredness scale scores with levothyroxine treatment compared with placebo.²¹ This finding was consistent even in the subgroup with a higher baseline symptom burden.²²

Continue to: Two small RCTs evaluated...

Two small RCTs evaluated treatment of SCH with depressive symptoms and cognitive function, neither finding benefit compared with placebo.^12,23 A 2018 systematic review and meta-analysis of 21 studies and 2192 adults did not show a benefit to quality of life or thyroid-specific symptoms in those treated for SCH compared with controls.²⁴

RCT support also is lacking for a reduction in cardiovascular mortality following treatment for SCH. A large population-level retrospective cohort from Denmark showed no difference in cardiovascular mortality or myocardial infarction in those treated for SCH compared with controls.²⁵ Pooled results from 2 RCTs (for patients older than 65 years, and those older than 80 years) showed no change in risk for cardiovascular outcomes in older adults treated for SCH.²⁶ Older adults treated for SCH in the TRUST trial showed no improvements in systolic or diastolic­ function on echocardiography.^27­ Two trials showed no difference in carotid intima-media thickness with treatment of SCH compared with placebo.^28,29

While most of the RCT data come from older adults, a retrospective cohort study in the United Kingdom of younger (ages 40-70 years; n = 3093) and older (age > 70 years; n = 1642) patients showed a reduction in cardiovascular mortality among treated patients who were younger (hazard ratio [HR] = 0.61; 4.2% vs. 6.6%) but not those who were older (HR = 0.99; 12.7% vs. 10.7%).³⁰ There is also evidence that thyroid size in those with goiter can be reduced with treatment of SCH.³¹

A measured approach to treating subclinical hypothyroidism

Consider several factors when deciding whether to treat SCH. For instance, RCT data suggest a lack of treatment benefit in relieving depression, improving cognition, or reducing general hypothyroid symptoms. Treatment of SCH in older adults does not appear to improve cardiovascular outcomes. The question of whether long-term treatment of SCH in younger patients reduces cardiovascular morbidity or mortality lacks answers from RCTs. Before diagnosing SCH or starting treatment, always confirm SCH with repeat testing in 2 to 3 months, as a high percentage of those with untreated SCH will have normal thyroid function on repeat testing.

Before diagnosing subclinical hypothyroidism (SCH) or starting treatment, always confirm SCH with repeat testing in 2 to 3 months.

In the event you and your patient elect to treat SCH, guidelines and trials generally support a low initial daily dose of 25 to 50 mcg of levothyroxine (T4), followed with dose changes­ every 4 to 8 weeks and a goal of normalizing TSH to within the lower half of the reference range (0.4-2.5 mIU/L).¹⁴ This is generally similar to published treatment goals for primary hypothyroidism and is based on studies suggesting the lower half of the reference range is normal for young, healthy, euthyroid individuals.³² Though full replacement doses (1.6-1.8 mcg/kg of ideal body weight) can be started for those who are elderly or who have ischemic heart disease or angina, this approach should be avoided in favor of low-dose initial therapy.³³ Thyroid supplements are best absorbed when taken apart from food, calcium, or iron supplements. The ATA suggests taking thyroid medication 60 minutes before breakfast or at bedtime (3 or more hours after the evening meal).³³

Continue to: Screening guidelines differ

Lacking population-level screening data from RCTs, most organizations do not recommend screening for thyroid dysfunction or they note insufficient evidence to make a screening recommendation (TABLE 2^17,19,20,34). In their most recent recommendation statement on the subject in 2015, the US Preventive Services Task Force (USPSTF) concluded the current evidence was insufficient to recommend for or against thyroid dysfunction screening in nonpregnant, asymptomatic adults.¹⁷ This differs from the ATA and the American Association of Clinical Endocrinology (AACE; formerly known as the American Association of Clinical Endocrinologists), which both recommend targeted screening for thyroid dysfunction based on symptoms or risk factors.²⁰

What about subclinical hypothyroidism in pregnancy?

Overt hypothyroidism is associated with adverse events during pregnancy and with subsequent neurodevelopmental complications in children, although the effects of SCH during pregnancy remain less certain. Concerns have been raised over the potential association of SCH with pregnancy loss, placental abruption, premature rupture of membranes, and neonatal death.³⁵ Historically, the prevalence of SCH during pregnancy has ranged from 2% to 2.5%, but using lower trimester-based TSH reference ranges, the prevalence of SCH in pregnancy may be as high as 15%.³⁵

Guided by a large RCT that failed to find benefit (pregnancy outcomes, neurodevelopmental outcomes in children) following treatment of SCH in pregnancy,³⁶ the American College of Obstetricians and Gynecologists (ACOG) recommends against routine screening for thyroid disease in pregnancy.³⁴ The ATA notes insufficient evidence to rec-ommend universal screening for thyroid dysfunction in pregnancy but recommends targeted screening of those with risk factors.³⁷ Data are conflicting on the benefit of treating known or recently detected SCH on pregnancy outcomes including pregnancy loss.^35,38 As such, the American Society of Reproductive Medicine and the ATA both generally recommend treatment of SCH in pregnant patients, particularly when the TSH is ≥ 4.0 mIU/L and TPOAbs are present.^37,39

^a The ATA, ETA, and NICE have slightly different recommendations when a TSH level = 10 mIU/L. ETA and NICE recommend prioritizing treatment for individuals with this level, while ATA recommends treatment when individual factors are also considered.

ACKNOWLEDGEMENT
The authors thank Family Medicine Medical Librarian Gwen Wilson, MLS, AHIP, for her assistance with literature searches.

CORRESPONDENCE
Nicholas LeFevre, MD, Family and Community Medicine, University of Missouri–Columbia School of Medicine, One Hospital Drive, M224 Medical Science Building, Columbia, MO 65212; [email protected]

Subclinical hypothyroidism (SCH) is a biochemical state in which the thyroid-stimulating hormone (TSH) is elevated while the free thyroxine (T4) level is normal. Overt hypothyroidism is not diagnosed until the free T4 level is decreased, regardless of the degree of TSH elevation.

The overall prevalence of SCH in iodine-rich areas is 4% to 10%, with a risk for progression to overt hypothyroidism of between 2% and 6% annually.¹ The prevalence of SCH varies depending on the TSH reference range used.¹ The normal reference range for TSH varies depending on the laboratory and/or the reference population surveyed, with the range likely widening with increasing age.

SCH is most common among women, the elderly, and White individuals.² The discovery of SCH is often incidental, given that usually it is detected by laboratory findings alone without associated symptoms of overt hypothyroidism.³

The not-so-significant role of symptoms in subclinical hypothyroidism

Symptoms associated with overt hypothyroidism include constipation, dry skin, fatigue, slow thinking, poor memory, muscle cramps, weakness, and cold intolerance. In SCH, these symptoms are inconsistent, with around 1 in 3 patients having no symptoms at all.⁴

One study reported that roughly 18% of euthyroid individuals, 22% of SCH patients, and 26% of those with overt hypothyroidism reported 4 or more symptoms classically thought to be related to hypothyroidism.⁴ A large Danish cohort study found that hypothyroid symptoms were no more common in patients with SCH than in euthyroid individuals in the general population.⁵ These studies question the validity of attributing symptoms to SCH.

Adverse health associations

Observational data suggest that SCH is associated with an increased risk for dyslipidemia, coronary heart disease, heart failure, and cardiovascular mortality, particularly in those with TSH levels ≥ 10 mIU/L.^6,7 Such associations were not found for most adults with TSH levels between 5 and 10 mIU/L.⁸ There are also potential associations of SCH with obesity, nonalcoholic fatty liver disease, and nonalcoholic steatohepatitis.^9,10 Despite thyroid studies being commonly ordered as part of a mental health evaluation, SCH has not been statistically associated with depressive symptoms.^11,12

Caveats with laboratory testing

There are several issues to consider when performing a laboratory assessment of thyroid function. TSH levels fluctuate considerably during the day, as TSH secretion has a circadian rhythm. TSH values are 50% higher at night and in the early morning than during the rest of the day.¹³ TSH values also may rise in response to current illness or stress. Due to this biologic variability, repeat testing to confirm TSH levels is recommended if an initial test result is abnormal.¹⁴

Continue to: An exact reference range...

Supplements containing biotin should be withheld for several days before assessing thyroid function.

An exact reference range for TSH is not widely agreed upon—although most laboratories regard 4.0 to 5.0 mIU/L as the high-end cutoff for normal. Also, “normal” TSH levels appear to differ by age. Accordingly, some experts have recommended an age-based reference range for TSH levels,¹⁵ although this is not implemented widely by laboratories. A TSH level of 6.0 mIU/L (or even higher) may be more appropriate for adults older than 65 years.¹

Biotin supplementation has been shown to cause spurious thyroid testing results (TSH, T3, T4) depending on the type of assay used. Therefore, supplements containing biotin should be withheld for several days before assessing thyroid function.¹⁶Patients with SCH are often categorized as having TSH levels between 4.5 and 10 mIU/L (around 90% of patients) or levels ≥ 10 mIU/L.^8,17 If followed for 5 years, approximately 60% of patients with SCH and TSH levels between 4 and 10 mIU/L will normalize without intervention.¹⁸ Normalization is less common in patients with a TSH level greater than 10 mIU/L.¹⁸

The risk for progression to overt hypothyroidism also appears to be higher for those with certain risk factors. These include higher baseline TSH levels, presence of thyroid peroxidase antibodies (TPOAbs), or history of neck irradiation or radioactive iodine uptake.¹ Other risk factors for eventual thyroid dysfunction include female sex, older age, goiter, and high iodine intake.¹³

Evidence for treatment varies

Guidelines for the treatment of SCH (TABLE 1^8,14,19,20) are founded on the condition’s risk for progression to overt hypothyroidism and its association with health consequences such as cardiovascular disease. Guidelines of the American Thyroid Association (ATA) and European Thyroid Association (ETA), and those of the United Kingdom–based National Institute for Health and Care Excellence (NICE), prioritize treatment for individuals with a TSH level > 10 mIU/L^a and for those with TSH values < 10 mIU/L but still elevated and apparent symptoms of hypothyroidism.^14,19,20 The strength of evidence behind this guidance is challenged by a lack of data from prospective randomized controlled trials (RCTs) demonstrating health benefits following treatment of SCH. The British Medical Journal (BMJ) Guideline cites this lack of evidence and recommends against treating SCH at any TSH level, regardless of symptoms.⁸

There are few large RCTs of treatment outcomes for SCH. A 2017 RCT (the Thyroid Hormone Replacement for Untreated Older Adults with Subclinical Hypothyroidism, or TRUST, trial) of 737 adults older than 65 years with SCH evaluated the ability of levothyroxine to normalize TSH values compared with placebo. At 1 year, there was no difference in hypothyroid symptoms or tiredness scale scores with levothyroxine treatment compared with placebo.²¹ This finding was consistent even in the subgroup with a higher baseline symptom burden.²²

Continue to: Two small RCTs evaluated...

Two small RCTs evaluated treatment of SCH with depressive symptoms and cognitive function, neither finding benefit compared with placebo.^12,23 A 2018 systematic review and meta-analysis of 21 studies and 2192 adults did not show a benefit to quality of life or thyroid-specific symptoms in those treated for SCH compared with controls.²⁴

RCT support also is lacking for a reduction in cardiovascular mortality following treatment for SCH. A large population-level retrospective cohort from Denmark showed no difference in cardiovascular mortality or myocardial infarction in those treated for SCH compared with controls.²⁵ Pooled results from 2 RCTs (for patients older than 65 years, and those older than 80 years) showed no change in risk for cardiovascular outcomes in older adults treated for SCH.²⁶ Older adults treated for SCH in the TRUST trial showed no improvements in systolic or diastolic­ function on echocardiography.^27­ Two trials showed no difference in carotid intima-media thickness with treatment of SCH compared with placebo.^28,29

While most of the RCT data come from older adults, a retrospective cohort study in the United Kingdom of younger (ages 40-70 years; n = 3093) and older (age > 70 years; n = 1642) patients showed a reduction in cardiovascular mortality among treated patients who were younger (hazard ratio [HR] = 0.61; 4.2% vs. 6.6%) but not those who were older (HR = 0.99; 12.7% vs. 10.7%).³⁰ There is also evidence that thyroid size in those with goiter can be reduced with treatment of SCH.³¹

A measured approach to treating subclinical hypothyroidism

Consider several factors when deciding whether to treat SCH. For instance, RCT data suggest a lack of treatment benefit in relieving depression, improving cognition, or reducing general hypothyroid symptoms. Treatment of SCH in older adults does not appear to improve cardiovascular outcomes. The question of whether long-term treatment of SCH in younger patients reduces cardiovascular morbidity or mortality lacks answers from RCTs. Before diagnosing SCH or starting treatment, always confirm SCH with repeat testing in 2 to 3 months, as a high percentage of those with untreated SCH will have normal thyroid function on repeat testing.

Before diagnosing subclinical hypothyroidism (SCH) or starting treatment, always confirm SCH with repeat testing in 2 to 3 months.

In the event you and your patient elect to treat SCH, guidelines and trials generally support a low initial daily dose of 25 to 50 mcg of levothyroxine (T4), followed with dose changes­ every 4 to 8 weeks and a goal of normalizing TSH to within the lower half of the reference range (0.4-2.5 mIU/L).¹⁴ This is generally similar to published treatment goals for primary hypothyroidism and is based on studies suggesting the lower half of the reference range is normal for young, healthy, euthyroid individuals.³² Though full replacement doses (1.6-1.8 mcg/kg of ideal body weight) can be started for those who are elderly or who have ischemic heart disease or angina, this approach should be avoided in favor of low-dose initial therapy.³³ Thyroid supplements are best absorbed when taken apart from food, calcium, or iron supplements. The ATA suggests taking thyroid medication 60 minutes before breakfast or at bedtime (3 or more hours after the evening meal).³³

Continue to: Screening guidelines differ

Lacking population-level screening data from RCTs, most organizations do not recommend screening for thyroid dysfunction or they note insufficient evidence to make a screening recommendation (TABLE 2^17,19,20,34). In their most recent recommendation statement on the subject in 2015, the US Preventive Services Task Force (USPSTF) concluded the current evidence was insufficient to recommend for or against thyroid dysfunction screening in nonpregnant, asymptomatic adults.¹⁷ This differs from the ATA and the American Association of Clinical Endocrinology (AACE; formerly known as the American Association of Clinical Endocrinologists), which both recommend targeted screening for thyroid dysfunction based on symptoms or risk factors.²⁰

What about subclinical hypothyroidism in pregnancy?

Overt hypothyroidism is associated with adverse events during pregnancy and with subsequent neurodevelopmental complications in children, although the effects of SCH during pregnancy remain less certain. Concerns have been raised over the potential association of SCH with pregnancy loss, placental abruption, premature rupture of membranes, and neonatal death.³⁵ Historically, the prevalence of SCH during pregnancy has ranged from 2% to 2.5%, but using lower trimester-based TSH reference ranges, the prevalence of SCH in pregnancy may be as high as 15%.³⁵

Guided by a large RCT that failed to find benefit (pregnancy outcomes, neurodevelopmental outcomes in children) following treatment of SCH in pregnancy,³⁶ the American College of Obstetricians and Gynecologists (ACOG) recommends against routine screening for thyroid disease in pregnancy.³⁴ The ATA notes insufficient evidence to rec-ommend universal screening for thyroid dysfunction in pregnancy but recommends targeted screening of those with risk factors.³⁷ Data are conflicting on the benefit of treating known or recently detected SCH on pregnancy outcomes including pregnancy loss.^35,38 As such, the American Society of Reproductive Medicine and the ATA both generally recommend treatment of SCH in pregnant patients, particularly when the TSH is ≥ 4.0 mIU/L and TPOAbs are present.^37,39

^a The ATA, ETA, and NICE have slightly different recommendations when a TSH level = 10 mIU/L. ETA and NICE recommend prioritizing treatment for individuals with this level, while ATA recommends treatment when individual factors are also considered.

ACKNOWLEDGEMENT
The authors thank Family Medicine Medical Librarian Gwen Wilson, MLS, AHIP, for her assistance with literature searches.

CORRESPONDENCE
Nicholas LeFevre, MD, Family and Community Medicine, University of Missouri–Columbia School of Medicine, One Hospital Drive, M224 Medical Science Building, Columbia, MO 65212; [email protected]

Subclinical hypothyroidism (SCH) is a biochemical state in which the thyroid-stimulating hormone (TSH) is elevated while the free thyroxine (T4) level is normal. Overt hypothyroidism is not diagnosed until the free T4 level is decreased, regardless of the degree of TSH elevation.

The overall prevalence of SCH in iodine-rich areas is 4% to 10%, with a risk for progression to overt hypothyroidism of between 2% and 6% annually.¹ The prevalence of SCH varies depending on the TSH reference range used.¹ The normal reference range for TSH varies depending on the laboratory and/or the reference population surveyed, with the range likely widening with increasing age.

SCH is most common among women, the elderly, and White individuals.² The discovery of SCH is often incidental, given that usually it is detected by laboratory findings alone without associated symptoms of overt hypothyroidism.³

The not-so-significant role of symptoms in subclinical hypothyroidism

Symptoms associated with overt hypothyroidism include constipation, dry skin, fatigue, slow thinking, poor memory, muscle cramps, weakness, and cold intolerance. In SCH, these symptoms are inconsistent, with around 1 in 3 patients having no symptoms at all.⁴

One study reported that roughly 18% of euthyroid individuals, 22% of SCH patients, and 26% of those with overt hypothyroidism reported 4 or more symptoms classically thought to be related to hypothyroidism.⁴ A large Danish cohort study found that hypothyroid symptoms were no more common in patients with SCH than in euthyroid individuals in the general population.⁵ These studies question the validity of attributing symptoms to SCH.

Adverse health associations

Observational data suggest that SCH is associated with an increased risk for dyslipidemia, coronary heart disease, heart failure, and cardiovascular mortality, particularly in those with TSH levels ≥ 10 mIU/L.^6,7 Such associations were not found for most adults with TSH levels between 5 and 10 mIU/L.⁸ There are also potential associations of SCH with obesity, nonalcoholic fatty liver disease, and nonalcoholic steatohepatitis.^9,10 Despite thyroid studies being commonly ordered as part of a mental health evaluation, SCH has not been statistically associated with depressive symptoms.^11,12

Caveats with laboratory testing

There are several issues to consider when performing a laboratory assessment of thyroid function. TSH levels fluctuate considerably during the day, as TSH secretion has a circadian rhythm. TSH values are 50% higher at night and in the early morning than during the rest of the day.¹³ TSH values also may rise in response to current illness or stress. Due to this biologic variability, repeat testing to confirm TSH levels is recommended if an initial test result is abnormal.¹⁴

Continue to: An exact reference range...

Supplements containing biotin should be withheld for several days before assessing thyroid function.

An exact reference range for TSH is not widely agreed upon—although most laboratories regard 4.0 to 5.0 mIU/L as the high-end cutoff for normal. Also, “normal” TSH levels appear to differ by age. Accordingly, some experts have recommended an age-based reference range for TSH levels,¹⁵ although this is not implemented widely by laboratories. A TSH level of 6.0 mIU/L (or even higher) may be more appropriate for adults older than 65 years.¹

Biotin supplementation has been shown to cause spurious thyroid testing results (TSH, T3, T4) depending on the type of assay used. Therefore, supplements containing biotin should be withheld for several days before assessing thyroid function.¹⁶Patients with SCH are often categorized as having TSH levels between 4.5 and 10 mIU/L (around 90% of patients) or levels ≥ 10 mIU/L.^8,17 If followed for 5 years, approximately 60% of patients with SCH and TSH levels between 4 and 10 mIU/L will normalize without intervention.¹⁸ Normalization is less common in patients with a TSH level greater than 10 mIU/L.¹⁸

The risk for progression to overt hypothyroidism also appears to be higher for those with certain risk factors. These include higher baseline TSH levels, presence of thyroid peroxidase antibodies (TPOAbs), or history of neck irradiation or radioactive iodine uptake.¹ Other risk factors for eventual thyroid dysfunction include female sex, older age, goiter, and high iodine intake.¹³

Evidence for treatment varies

Guidelines for the treatment of SCH (TABLE 1^8,14,19,20) are founded on the condition’s risk for progression to overt hypothyroidism and its association with health consequences such as cardiovascular disease. Guidelines of the American Thyroid Association (ATA) and European Thyroid Association (ETA), and those of the United Kingdom–based National Institute for Health and Care Excellence (NICE), prioritize treatment for individuals with a TSH level > 10 mIU/L^a and for those with TSH values < 10 mIU/L but still elevated and apparent symptoms of hypothyroidism.^14,19,20 The strength of evidence behind this guidance is challenged by a lack of data from prospective randomized controlled trials (RCTs) demonstrating health benefits following treatment of SCH. The British Medical Journal (BMJ) Guideline cites this lack of evidence and recommends against treating SCH at any TSH level, regardless of symptoms.⁸

There are few large RCTs of treatment outcomes for SCH. A 2017 RCT (the Thyroid Hormone Replacement for Untreated Older Adults with Subclinical Hypothyroidism, or TRUST, trial) of 737 adults older than 65 years with SCH evaluated the ability of levothyroxine to normalize TSH values compared with placebo. At 1 year, there was no difference in hypothyroid symptoms or tiredness scale scores with levothyroxine treatment compared with placebo.²¹ This finding was consistent even in the subgroup with a higher baseline symptom burden.²²

Continue to: Two small RCTs evaluated...

Two small RCTs evaluated treatment of SCH with depressive symptoms and cognitive function, neither finding benefit compared with placebo.^12,23 A 2018 systematic review and meta-analysis of 21 studies and 2192 adults did not show a benefit to quality of life or thyroid-specific symptoms in those treated for SCH compared with controls.²⁴

RCT support also is lacking for a reduction in cardiovascular mortality following treatment for SCH. A large population-level retrospective cohort from Denmark showed no difference in cardiovascular mortality or myocardial infarction in those treated for SCH compared with controls.²⁵ Pooled results from 2 RCTs (for patients older than 65 years, and those older than 80 years) showed no change in risk for cardiovascular outcomes in older adults treated for SCH.²⁶ Older adults treated for SCH in the TRUST trial showed no improvements in systolic or diastolic­ function on echocardiography.^27­ Two trials showed no difference in carotid intima-media thickness with treatment of SCH compared with placebo.^28,29

While most of the RCT data come from older adults, a retrospective cohort study in the United Kingdom of younger (ages 40-70 years; n = 3093) and older (age > 70 years; n = 1642) patients showed a reduction in cardiovascular mortality among treated patients who were younger (hazard ratio [HR] = 0.61; 4.2% vs. 6.6%) but not those who were older (HR = 0.99; 12.7% vs. 10.7%).³⁰ There is also evidence that thyroid size in those with goiter can be reduced with treatment of SCH.³¹

A measured approach to treating subclinical hypothyroidism

Consider several factors when deciding whether to treat SCH. For instance, RCT data suggest a lack of treatment benefit in relieving depression, improving cognition, or reducing general hypothyroid symptoms. Treatment of SCH in older adults does not appear to improve cardiovascular outcomes. The question of whether long-term treatment of SCH in younger patients reduces cardiovascular morbidity or mortality lacks answers from RCTs. Before diagnosing SCH or starting treatment, always confirm SCH with repeat testing in 2 to 3 months, as a high percentage of those with untreated SCH will have normal thyroid function on repeat testing.

Before diagnosing subclinical hypothyroidism (SCH) or starting treatment, always confirm SCH with repeat testing in 2 to 3 months.

In the event you and your patient elect to treat SCH, guidelines and trials generally support a low initial daily dose of 25 to 50 mcg of levothyroxine (T4), followed with dose changes­ every 4 to 8 weeks and a goal of normalizing TSH to within the lower half of the reference range (0.4-2.5 mIU/L).¹⁴ This is generally similar to published treatment goals for primary hypothyroidism and is based on studies suggesting the lower half of the reference range is normal for young, healthy, euthyroid individuals.³² Though full replacement doses (1.6-1.8 mcg/kg of ideal body weight) can be started for those who are elderly or who have ischemic heart disease or angina, this approach should be avoided in favor of low-dose initial therapy.³³ Thyroid supplements are best absorbed when taken apart from food, calcium, or iron supplements. The ATA suggests taking thyroid medication 60 minutes before breakfast or at bedtime (3 or more hours after the evening meal).³³

Continue to: Screening guidelines differ

Lacking population-level screening data from RCTs, most organizations do not recommend screening for thyroid dysfunction or they note insufficient evidence to make a screening recommendation (TABLE 2^17,19,20,34). In their most recent recommendation statement on the subject in 2015, the US Preventive Services Task Force (USPSTF) concluded the current evidence was insufficient to recommend for or against thyroid dysfunction screening in nonpregnant, asymptomatic adults.¹⁷ This differs from the ATA and the American Association of Clinical Endocrinology (AACE; formerly known as the American Association of Clinical Endocrinologists), which both recommend targeted screening for thyroid dysfunction based on symptoms or risk factors.²⁰

What about subclinical hypothyroidism in pregnancy?

Overt hypothyroidism is associated with adverse events during pregnancy and with subsequent neurodevelopmental complications in children, although the effects of SCH during pregnancy remain less certain. Concerns have been raised over the potential association of SCH with pregnancy loss, placental abruption, premature rupture of membranes, and neonatal death.³⁵ Historically, the prevalence of SCH during pregnancy has ranged from 2% to 2.5%, but using lower trimester-based TSH reference ranges, the prevalence of SCH in pregnancy may be as high as 15%.³⁵

Guided by a large RCT that failed to find benefit (pregnancy outcomes, neurodevelopmental outcomes in children) following treatment of SCH in pregnancy,³⁶ the American College of Obstetricians and Gynecologists (ACOG) recommends against routine screening for thyroid disease in pregnancy.³⁴ The ATA notes insufficient evidence to rec-ommend universal screening for thyroid dysfunction in pregnancy but recommends targeted screening of those with risk factors.³⁷ Data are conflicting on the benefit of treating known or recently detected SCH on pregnancy outcomes including pregnancy loss.^35,38 As such, the American Society of Reproductive Medicine and the ATA both generally recommend treatment of SCH in pregnant patients, particularly when the TSH is ≥ 4.0 mIU/L and TPOAbs are present.^37,39

^a The ATA, ETA, and NICE have slightly different recommendations when a TSH level = 10 mIU/L. ETA and NICE recommend prioritizing treatment for individuals with this level, while ATA recommends treatment when individual factors are also considered.

ACKNOWLEDGEMENT
The authors thank Family Medicine Medical Librarian Gwen Wilson, MLS, AHIP, for her assistance with literature searches.

CORRESPONDENCE
Nicholas LeFevre, MD, Family and Community Medicine, University of Missouri–Columbia School of Medicine, One Hospital Drive, M224 Medical Science Building, Columbia, MO 65212; [email protected]