Reverse T3 or perverse T3? Still puzzling after 40 years

Four decades after reverse T₃ (3,3´5´-triiodothyronine) was discovered, its physiologic and clinical relevance remains unclear and is still being studied. But scientific uncertainty has not stopped writers in the consumer press and on the Internet from making unsubstantiated claims about this hormone. Many patients believe their hypothyroid symptoms are due to high levels of reverse T₃ and want to be tested for it, and some even bring in test results from independent laboratories.  

HOW THYROID HORMONES WERE DISCOVERED

The 20th century saw important advances in knowledge of the biochemistry of thyroid hormones (Figure 1),^1–18 such as the isolation of thyroxine (T₄) by Kendall¹ in 1915 and its synthesis by Harington and Barger³ in 1927. Another milestone was the isolation and synthesis of triiodothyronine (T₃) by Gross and Pitt-Rivers⁵ in 1953. In 1955, Pitt-Rivers et al⁶ suggested that T₃ is formed in vivo from conversion of T₄, but this theory remained unproven in humans at that time.

In 1970, Braverman et al⁹ showed that T₄ is converted to T₃ in athyreotic humans, and Sterling et al¹⁰ demonstrated the same in healthy humans. During that decade, techniques for measuring T₄ were refined,¹¹ and a specific radioimmunoassay for reverse T₃ allowed a glimpse of its physiologic role.¹² In 1975, Chopra et al¹³ noted reciprocal changes in the levels of T₃ and reverse T₃ in systemic illnesses—ie, when people are sick, their T₃ levels go down and their reverse T₃ levels go up.

In 1977, Burman et al¹⁷ developed a radioimmunoassay for reverse T₃ that confirmed its presence in the serum of normal humans. Further, they showed that serum reverse T₃ levels were low in hypothyroid patients and in athyreotic patients receiving low daily doses of levothyroxine. Conversely, reverse T₃ levels were high in hyperthyroid patients and in athyreotic patients receiving high doses of levothyroxine (Figure 2).¹⁷

The end of the 70s was marked by a surge of interest in T₄ metabolites, including the development of a radioimmunoassay for 3,3´-diiodothyronine (3-3´ T₂).¹⁸

The observed reciprocal changes in serum levels of T₃ and reverse T₃ suggested that T₄ degradation is regulated into activating (T₃) or inactivating (reverse T₃) pathways, and that these changes are a presumed homeostatic process of energy conservation.¹⁹

HOW THYROID HORMONES ARE METABOLIZED

In the thyroid gland, for thyroid hormones to be synthesized, iodide must be oxidized and incorporated into the precursors 3-monoiodotyrosine (MIT) and 3,5-diiodotyrosine (DIT). This process is mediated by the enzyme thyroid peroxidase in the presence of hydrogen peroxide.²⁰

The thyroid can make T₄ and some T₃

T₄ is the main iodothyronine produced by the thyroid gland, at a rate of 80 to 100 µg per day.²¹ It is synthesized from the fusion of 2 DIT molecules.

The thyroid can also make T₃ by fusing 1 DIT and 1 MIT molecule, but this process accounts for no more than 20% of the circulating T₃ in humans. The rest of T₃, and 95% to 98% of all reverse T₃, is derived from peripheral conversion of T₄ through deiodination.

T₄ is converted to T₃ or reverse T₃

The metabolic transformation of thyroid hormones in peripheral tissues determines their biologic potency and regulates their biologic effects.

The number 4 in T₄ means it has 4 iodine atoms. It can lose 1 of them, yielding either T₃ or reverse T₃, depending on which iodine atom it loses (Figure 3). Loss of iodine from the five-prime (5´) position on its outer ring yields T₃, the most potent thyroid hormone, produced at a rate of 30 to 40 µg per day.²¹ On the other hand, when T₄ loses an iodine atom from the five (5) position on its inner ring it yields reverse T₃, produced at a rate slightly less than that of T₃, 28 to 40 µg per day.²¹ Reverse T₃ is inactive.

Both T₃ and reverse T₃ can shed more iodine atoms, forming in turn various isomers of T₂, T₁, and ultimately T₀. Other pathways for thyroid hormone metabolism include glucuronidation, sulfation, oxidative deamination, and ether bond cleavage.^20–22

D1 and D2 catalyze T₃, D3 catalyzes reverse T₃

Three types of enzymes that mediate deiodination have been identified and designated D1, D2, and D3. In humans they are expressed in variable amounts throughout the body:

• D1 mainly in the liver, kidneys, thyroid, and pituitary, but notably absent in the central nervous system
• D2 in the central nervous system, pituitary, brown adipose tissue, thyroid, placenta, skeletal muscle, and heart
• D3 in the central nervous system, skin, hemangiomas, fetal liver, placenta, and fetal tissues.²³

D1 and D2 are responsible for converting T₄ to T₃, and D3 is responsible for converting T₄ to reverse T₃.

Plasma concentrations of free T₄ and free T₃ are relatively constant; however, tissue concentrations of free T₃ vary in different tissues according to the amount of hormone transported and the activity of local deiodinases.²³ Most thyroid hormone actions are initiated after T₃ binds to its nuclear receptor. In this setting, deiodinases play a critical role in maintaining tissue and cellular thyroid hormone levels, so that thyroid hormone signaling can change irrespective of serum hormonal concentrations.^22–24 For example, in the central nervous system, production of T₃ by local D2 is significantly relevant for T₃ homeostasis.^22,23

Deiodinases also modulate the tissue-specific concentrations of T₃ in response to iodine deficiency and to changes in thyroid state.²³ During iodine deficiency and hypothyroidism, tissues that express D2, especially brain tissues, increase the activity of this enzyme in order to increase local conversion of T₄ to T₃. In hyperthyroidism, D1 overexpression contributes to the relative excess of T₃ production, while D3 up-regulation in the brain protects the central nervous system from excessive amounts of thyroid hormone.²³

 

 

REVERSE T₃ AND SYSTEMIC ILLNESS

D3 is the main physiologic inactivator of thyroid hormones. This enzyme plays a central role in protecting tissues from an excess of thyroid hormone.^23,24 This mechanism is crucial for fetal development and explains the high expression of D3 in the human placenta and fetal tissues.

In adult tissues, the importance of D3 in the regulation of thyroid hormone homeostasis becomes apparent under certain pathophysiologic conditions, such as nonthyroidal illness and malnutrition.

Whenever a reduction in metabolism is homeostatically desirable, such as in critically ill patients or during starvation, conversion to T₃ is reduced and, alternatively, conversion to reverse T₃ is increased. This pathway represents a metabolic adaptation that may protect the tissues from the catabolic effects of thyroid hormone that could otherwise worsen the patient’s basic clinical condition.

Euthyroid sick syndrome or hypothyroid?

In a variety of systemic illnesses, some patients with low T₃, low or normal T₄, and normal thyroid-stimulating hormone (TSH) levels could in fact be “sick euthyroid” rather than hypothyroid. The first reports of the euthyroid sick syndrome or low T₃ syndrome date back to about 1976, and even though assays for reverse T₃ were not widely available, some authors linked the syndrome to high levels of reverse T₃.^15,16 The syndrome is also known as nonthyroidal illness syndrome.

Advances in techniques for measuring T₃, reverse T₃, and other iodothyronines filled a gap in the understanding of the alterations that occur in thyroid hormone economy during severe nonthyroidal diseases. In 1982, Wartofsky and Burman²⁵ reviewed the alterations in thyroid function in patients with systemic illness and discussed other factors that may alter thyroid economy, such as age, stress, and diverse drugs.

More recently, the low-T₃ syndrome was revisited with a generalized concept regarding the role of D3 in the syndrome.²⁶ D3, normally undetectable in mature tissues, is reactivated in diverse cell types in response to injury and is responsible for a fall in serum T₃ levels. Hypoxia induces D3 activity and mRNA in vitro and in vivo.²⁷ Recent studies have focused on the role of cytokines in the low T₃ syndrome. For instance, interleukin 6 reduces D1 and D2 activity and increases D3 activity in vitro.²⁸

In the outpatient setting, diverse conditions may affect thyroid hormone homeostasis, compatible with mild or atypical forms of low-T₃ syndrome, including caloric deprivation, heart failure, and human immunodeficiency virus infection.²⁹

POSSIBLE CLINICAL UTILITY OF MEASURING REVERSE T₃

In inpatients

Unfortunately, measuring serum reverse T₃ levels has not, in general, proven clinically useful for the diagnosis of hypothyroidism in systemically ill patients. Burmeister³⁰ demonstrated, in a retrospective study, that when illness complicates the interpretation of thyroid function tests, serum reverse T₃ measurements do not reliably distinguish the hypothyroid sick patient from the euthyroid sick patient. The best way to make the diagnosis, Burmeister suggested, is by clinical assessment, combined use of free T₄ and TSH measurements, and patient follow-up.

Indeed, few clinical situations require measurement of reverse T₃ levels. We believe it can potentially be used to help the differential diagnosis between hypothyroidism and euthyroid sick syndrome. Reverse T₃ should always be analyzed in combination with TSH, T₃, and free T₄ with consideration of the patient’s clinical context. Table 1 helps to interpret the results. However, even in these circumstances, serum reverse T₃ levels are not always reliable, as demonstrated by Burmeister.³⁰ Another situation, even rarer, is in children or adults with massive hemangiomas. These tumors express high levels of D3 that can cause hypothyroidism.³¹

In the outpatient setting, the utility of reverse T3 measurements is controversial. In intensive care units, the differential diagnosis between hypothyroidism and nonthyroidal illness syndrome can sometimes be difficult. Reverse T₃ levels can be low, normal, or high regardless of the thyroidal state of the patient.³⁰ Moreover, endogenous changes in the hypothalamic-pituitary-thyroid axis may be further complicated by medications commonly used in intensive care units, such as dopamine and glucocorticoids. Changes in thyroid function should be evaluated in the context of the patient’s clinical condition (Table 1).²⁰ But regardless of the T₃ level, treatment with T₃ or T₄ should not be started without taking into consideration the patient’s general clinical context; controlled trials have not shown such therapy to be beneficial.²⁰

In outpatients

In noncritical conditions that may be associated with mild forms of low T₃ syndrome, patients generally present with low T₃ concentrations concurrently with low or normal TSH. Not infrequently, however, patients present with a serum reverse T₃ measurement and impute their symptoms of hypothyroidism to “abnormal” reverse T₃ levels, in spite of normal TSH levels.

There is no rationale for measuring reverse T₃ to initiate or to adjust levothyroxine therapy—the single test relevant for these purposes is the TSH measurement. The risks of basing treatment decisions on reverse T₃ levels include the use of excessive doses of levothyroxine that may lead to a state of subclinical or even clinical hyperthyroidism.

TAKE-HOME MESSAGE

The existence of an inactivating pathway of thyroid hormones represents a homeostatic mechanism, and in selected circumstances measuring serum reverse T₃ may be useful, such as in euthyroid sick patients. The discovery of the molecular mechanisms that lead to the reactivation of D3 in illness is an important field of research.

Four decades after reverse T₃ (3,3´5´-triiodothyronine) was discovered, its physiologic and clinical relevance remains unclear and is still being studied. But scientific uncertainty has not stopped writers in the consumer press and on the Internet from making unsubstantiated claims about this hormone. Many patients believe their hypothyroid symptoms are due to high levels of reverse T₃ and want to be tested for it, and some even bring in test results from independent laboratories.  

HOW THYROID HORMONES WERE DISCOVERED

The 20th century saw important advances in knowledge of the biochemistry of thyroid hormones (Figure 1),^1–18 such as the isolation of thyroxine (T₄) by Kendall¹ in 1915 and its synthesis by Harington and Barger³ in 1927. Another milestone was the isolation and synthesis of triiodothyronine (T₃) by Gross and Pitt-Rivers⁵ in 1953. In 1955, Pitt-Rivers et al⁶ suggested that T₃ is formed in vivo from conversion of T₄, but this theory remained unproven in humans at that time.

In 1970, Braverman et al⁹ showed that T₄ is converted to T₃ in athyreotic humans, and Sterling et al¹⁰ demonstrated the same in healthy humans. During that decade, techniques for measuring T₄ were refined,¹¹ and a specific radioimmunoassay for reverse T₃ allowed a glimpse of its physiologic role.¹² In 1975, Chopra et al¹³ noted reciprocal changes in the levels of T₃ and reverse T₃ in systemic illnesses—ie, when people are sick, their T₃ levels go down and their reverse T₃ levels go up.

In 1977, Burman et al¹⁷ developed a radioimmunoassay for reverse T₃ that confirmed its presence in the serum of normal humans. Further, they showed that serum reverse T₃ levels were low in hypothyroid patients and in athyreotic patients receiving low daily doses of levothyroxine. Conversely, reverse T₃ levels were high in hyperthyroid patients and in athyreotic patients receiving high doses of levothyroxine (Figure 2).¹⁷

The end of the 70s was marked by a surge of interest in T₄ metabolites, including the development of a radioimmunoassay for 3,3´-diiodothyronine (3-3´ T₂).¹⁸

The observed reciprocal changes in serum levels of T₃ and reverse T₃ suggested that T₄ degradation is regulated into activating (T₃) or inactivating (reverse T₃) pathways, and that these changes are a presumed homeostatic process of energy conservation.¹⁹

HOW THYROID HORMONES ARE METABOLIZED

In the thyroid gland, for thyroid hormones to be synthesized, iodide must be oxidized and incorporated into the precursors 3-monoiodotyrosine (MIT) and 3,5-diiodotyrosine (DIT). This process is mediated by the enzyme thyroid peroxidase in the presence of hydrogen peroxide.²⁰

The thyroid can make T₄ and some T₃

T₄ is the main iodothyronine produced by the thyroid gland, at a rate of 80 to 100 µg per day.²¹ It is synthesized from the fusion of 2 DIT molecules.

The thyroid can also make T₃ by fusing 1 DIT and 1 MIT molecule, but this process accounts for no more than 20% of the circulating T₃ in humans. The rest of T₃, and 95% to 98% of all reverse T₃, is derived from peripheral conversion of T₄ through deiodination.

T₄ is converted to T₃ or reverse T₃

The metabolic transformation of thyroid hormones in peripheral tissues determines their biologic potency and regulates their biologic effects.

The number 4 in T₄ means it has 4 iodine atoms. It can lose 1 of them, yielding either T₃ or reverse T₃, depending on which iodine atom it loses (Figure 3). Loss of iodine from the five-prime (5´) position on its outer ring yields T₃, the most potent thyroid hormone, produced at a rate of 30 to 40 µg per day.²¹ On the other hand, when T₄ loses an iodine atom from the five (5) position on its inner ring it yields reverse T₃, produced at a rate slightly less than that of T₃, 28 to 40 µg per day.²¹ Reverse T₃ is inactive.

Both T₃ and reverse T₃ can shed more iodine atoms, forming in turn various isomers of T₂, T₁, and ultimately T₀. Other pathways for thyroid hormone metabolism include glucuronidation, sulfation, oxidative deamination, and ether bond cleavage.^20–22

D1 and D2 catalyze T₃, D3 catalyzes reverse T₃

Three types of enzymes that mediate deiodination have been identified and designated D1, D2, and D3. In humans they are expressed in variable amounts throughout the body:

• D1 mainly in the liver, kidneys, thyroid, and pituitary, but notably absent in the central nervous system
• D2 in the central nervous system, pituitary, brown adipose tissue, thyroid, placenta, skeletal muscle, and heart
• D3 in the central nervous system, skin, hemangiomas, fetal liver, placenta, and fetal tissues.²³

D1 and D2 are responsible for converting T₄ to T₃, and D3 is responsible for converting T₄ to reverse T₃.

Plasma concentrations of free T₄ and free T₃ are relatively constant; however, tissue concentrations of free T₃ vary in different tissues according to the amount of hormone transported and the activity of local deiodinases.²³ Most thyroid hormone actions are initiated after T₃ binds to its nuclear receptor. In this setting, deiodinases play a critical role in maintaining tissue and cellular thyroid hormone levels, so that thyroid hormone signaling can change irrespective of serum hormonal concentrations.^22–24 For example, in the central nervous system, production of T₃ by local D2 is significantly relevant for T₃ homeostasis.^22,23

Deiodinases also modulate the tissue-specific concentrations of T₃ in response to iodine deficiency and to changes in thyroid state.²³ During iodine deficiency and hypothyroidism, tissues that express D2, especially brain tissues, increase the activity of this enzyme in order to increase local conversion of T₄ to T₃. In hyperthyroidism, D1 overexpression contributes to the relative excess of T₃ production, while D3 up-regulation in the brain protects the central nervous system from excessive amounts of thyroid hormone.²³

 

 

REVERSE T₃ AND SYSTEMIC ILLNESS

D3 is the main physiologic inactivator of thyroid hormones. This enzyme plays a central role in protecting tissues from an excess of thyroid hormone.^23,24 This mechanism is crucial for fetal development and explains the high expression of D3 in the human placenta and fetal tissues.

In adult tissues, the importance of D3 in the regulation of thyroid hormone homeostasis becomes apparent under certain pathophysiologic conditions, such as nonthyroidal illness and malnutrition.

Whenever a reduction in metabolism is homeostatically desirable, such as in critically ill patients or during starvation, conversion to T₃ is reduced and, alternatively, conversion to reverse T₃ is increased. This pathway represents a metabolic adaptation that may protect the tissues from the catabolic effects of thyroid hormone that could otherwise worsen the patient’s basic clinical condition.

Euthyroid sick syndrome or hypothyroid?

In a variety of systemic illnesses, some patients with low T₃, low or normal T₄, and normal thyroid-stimulating hormone (TSH) levels could in fact be “sick euthyroid” rather than hypothyroid. The first reports of the euthyroid sick syndrome or low T₃ syndrome date back to about 1976, and even though assays for reverse T₃ were not widely available, some authors linked the syndrome to high levels of reverse T₃.^15,16 The syndrome is also known as nonthyroidal illness syndrome.

Advances in techniques for measuring T₃, reverse T₃, and other iodothyronines filled a gap in the understanding of the alterations that occur in thyroid hormone economy during severe nonthyroidal diseases. In 1982, Wartofsky and Burman²⁵ reviewed the alterations in thyroid function in patients with systemic illness and discussed other factors that may alter thyroid economy, such as age, stress, and diverse drugs.

More recently, the low-T₃ syndrome was revisited with a generalized concept regarding the role of D3 in the syndrome.²⁶ D3, normally undetectable in mature tissues, is reactivated in diverse cell types in response to injury and is responsible for a fall in serum T₃ levels. Hypoxia induces D3 activity and mRNA in vitro and in vivo.²⁷ Recent studies have focused on the role of cytokines in the low T₃ syndrome. For instance, interleukin 6 reduces D1 and D2 activity and increases D3 activity in vitro.²⁸

In the outpatient setting, diverse conditions may affect thyroid hormone homeostasis, compatible with mild or atypical forms of low-T₃ syndrome, including caloric deprivation, heart failure, and human immunodeficiency virus infection.²⁹

POSSIBLE CLINICAL UTILITY OF MEASURING REVERSE T₃

In inpatients

Unfortunately, measuring serum reverse T₃ levels has not, in general, proven clinically useful for the diagnosis of hypothyroidism in systemically ill patients. Burmeister³⁰ demonstrated, in a retrospective study, that when illness complicates the interpretation of thyroid function tests, serum reverse T₃ measurements do not reliably distinguish the hypothyroid sick patient from the euthyroid sick patient. The best way to make the diagnosis, Burmeister suggested, is by clinical assessment, combined use of free T₄ and TSH measurements, and patient follow-up.

Indeed, few clinical situations require measurement of reverse T₃ levels. We believe it can potentially be used to help the differential diagnosis between hypothyroidism and euthyroid sick syndrome. Reverse T₃ should always be analyzed in combination with TSH, T₃, and free T₄ with consideration of the patient’s clinical context. Table 1 helps to interpret the results. However, even in these circumstances, serum reverse T₃ levels are not always reliable, as demonstrated by Burmeister.³⁰ Another situation, even rarer, is in children or adults with massive hemangiomas. These tumors express high levels of D3 that can cause hypothyroidism.³¹

In the outpatient setting, the utility of reverse T3 measurements is controversial. In intensive care units, the differential diagnosis between hypothyroidism and nonthyroidal illness syndrome can sometimes be difficult. Reverse T₃ levels can be low, normal, or high regardless of the thyroidal state of the patient.³⁰ Moreover, endogenous changes in the hypothalamic-pituitary-thyroid axis may be further complicated by medications commonly used in intensive care units, such as dopamine and glucocorticoids. Changes in thyroid function should be evaluated in the context of the patient’s clinical condition (Table 1).²⁰ But regardless of the T₃ level, treatment with T₃ or T₄ should not be started without taking into consideration the patient’s general clinical context; controlled trials have not shown such therapy to be beneficial.²⁰

In outpatients

In noncritical conditions that may be associated with mild forms of low T₃ syndrome, patients generally present with low T₃ concentrations concurrently with low or normal TSH. Not infrequently, however, patients present with a serum reverse T₃ measurement and impute their symptoms of hypothyroidism to “abnormal” reverse T₃ levels, in spite of normal TSH levels.

There is no rationale for measuring reverse T₃ to initiate or to adjust levothyroxine therapy—the single test relevant for these purposes is the TSH measurement. The risks of basing treatment decisions on reverse T₃ levels include the use of excessive doses of levothyroxine that may lead to a state of subclinical or even clinical hyperthyroidism.

TAKE-HOME MESSAGE

The existence of an inactivating pathway of thyroid hormones represents a homeostatic mechanism, and in selected circumstances measuring serum reverse T₃ may be useful, such as in euthyroid sick patients. The discovery of the molecular mechanisms that lead to the reactivation of D3 in illness is an important field of research.

Four decades after reverse T₃ (3,3´5´-triiodothyronine) was discovered, its physiologic and clinical relevance remains unclear and is still being studied. But scientific uncertainty has not stopped writers in the consumer press and on the Internet from making unsubstantiated claims about this hormone. Many patients believe their hypothyroid symptoms are due to high levels of reverse T₃ and want to be tested for it, and some even bring in test results from independent laboratories.  

HOW THYROID HORMONES WERE DISCOVERED

The 20th century saw important advances in knowledge of the biochemistry of thyroid hormones (Figure 1),^1–18 such as the isolation of thyroxine (T₄) by Kendall¹ in 1915 and its synthesis by Harington and Barger³ in 1927. Another milestone was the isolation and synthesis of triiodothyronine (T₃) by Gross and Pitt-Rivers⁵ in 1953. In 1955, Pitt-Rivers et al⁶ suggested that T₃ is formed in vivo from conversion of T₄, but this theory remained unproven in humans at that time.

In 1970, Braverman et al⁹ showed that T₄ is converted to T₃ in athyreotic humans, and Sterling et al¹⁰ demonstrated the same in healthy humans. During that decade, techniques for measuring T₄ were refined,¹¹ and a specific radioimmunoassay for reverse T₃ allowed a glimpse of its physiologic role.¹² In 1975, Chopra et al¹³ noted reciprocal changes in the levels of T₃ and reverse T₃ in systemic illnesses—ie, when people are sick, their T₃ levels go down and their reverse T₃ levels go up.

In 1977, Burman et al¹⁷ developed a radioimmunoassay for reverse T₃ that confirmed its presence in the serum of normal humans. Further, they showed that serum reverse T₃ levels were low in hypothyroid patients and in athyreotic patients receiving low daily doses of levothyroxine. Conversely, reverse T₃ levels were high in hyperthyroid patients and in athyreotic patients receiving high doses of levothyroxine (Figure 2).¹⁷

The end of the 70s was marked by a surge of interest in T₄ metabolites, including the development of a radioimmunoassay for 3,3´-diiodothyronine (3-3´ T₂).¹⁸

The observed reciprocal changes in serum levels of T₃ and reverse T₃ suggested that T₄ degradation is regulated into activating (T₃) or inactivating (reverse T₃) pathways, and that these changes are a presumed homeostatic process of energy conservation.¹⁹

HOW THYROID HORMONES ARE METABOLIZED

In the thyroid gland, for thyroid hormones to be synthesized, iodide must be oxidized and incorporated into the precursors 3-monoiodotyrosine (MIT) and 3,5-diiodotyrosine (DIT). This process is mediated by the enzyme thyroid peroxidase in the presence of hydrogen peroxide.²⁰

The thyroid can make T₄ and some T₃

T₄ is the main iodothyronine produced by the thyroid gland, at a rate of 80 to 100 µg per day.²¹ It is synthesized from the fusion of 2 DIT molecules.

The thyroid can also make T₃ by fusing 1 DIT and 1 MIT molecule, but this process accounts for no more than 20% of the circulating T₃ in humans. The rest of T₃, and 95% to 98% of all reverse T₃, is derived from peripheral conversion of T₄ through deiodination.

T₄ is converted to T₃ or reverse T₃

The metabolic transformation of thyroid hormones in peripheral tissues determines their biologic potency and regulates their biologic effects.

The number 4 in T₄ means it has 4 iodine atoms. It can lose 1 of them, yielding either T₃ or reverse T₃, depending on which iodine atom it loses (Figure 3). Loss of iodine from the five-prime (5´) position on its outer ring yields T₃, the most potent thyroid hormone, produced at a rate of 30 to 40 µg per day.²¹ On the other hand, when T₄ loses an iodine atom from the five (5) position on its inner ring it yields reverse T₃, produced at a rate slightly less than that of T₃, 28 to 40 µg per day.²¹ Reverse T₃ is inactive.

Both T₃ and reverse T₃ can shed more iodine atoms, forming in turn various isomers of T₂, T₁, and ultimately T₀. Other pathways for thyroid hormone metabolism include glucuronidation, sulfation, oxidative deamination, and ether bond cleavage.^20–22

D1 and D2 catalyze T₃, D3 catalyzes reverse T₃

Three types of enzymes that mediate deiodination have been identified and designated D1, D2, and D3. In humans they are expressed in variable amounts throughout the body:

• D1 mainly in the liver, kidneys, thyroid, and pituitary, but notably absent in the central nervous system
• D2 in the central nervous system, pituitary, brown adipose tissue, thyroid, placenta, skeletal muscle, and heart
• D3 in the central nervous system, skin, hemangiomas, fetal liver, placenta, and fetal tissues.²³

D1 and D2 are responsible for converting T₄ to T₃, and D3 is responsible for converting T₄ to reverse T₃.

Plasma concentrations of free T₄ and free T₃ are relatively constant; however, tissue concentrations of free T₃ vary in different tissues according to the amount of hormone transported and the activity of local deiodinases.²³ Most thyroid hormone actions are initiated after T₃ binds to its nuclear receptor. In this setting, deiodinases play a critical role in maintaining tissue and cellular thyroid hormone levels, so that thyroid hormone signaling can change irrespective of serum hormonal concentrations.^22–24 For example, in the central nervous system, production of T₃ by local D2 is significantly relevant for T₃ homeostasis.^22,23

Deiodinases also modulate the tissue-specific concentrations of T₃ in response to iodine deficiency and to changes in thyroid state.²³ During iodine deficiency and hypothyroidism, tissues that express D2, especially brain tissues, increase the activity of this enzyme in order to increase local conversion of T₄ to T₃. In hyperthyroidism, D1 overexpression contributes to the relative excess of T₃ production, while D3 up-regulation in the brain protects the central nervous system from excessive amounts of thyroid hormone.²³

 

 

REVERSE T₃ AND SYSTEMIC ILLNESS

D3 is the main physiologic inactivator of thyroid hormones. This enzyme plays a central role in protecting tissues from an excess of thyroid hormone.^23,24 This mechanism is crucial for fetal development and explains the high expression of D3 in the human placenta and fetal tissues.

In adult tissues, the importance of D3 in the regulation of thyroid hormone homeostasis becomes apparent under certain pathophysiologic conditions, such as nonthyroidal illness and malnutrition.

Whenever a reduction in metabolism is homeostatically desirable, such as in critically ill patients or during starvation, conversion to T₃ is reduced and, alternatively, conversion to reverse T₃ is increased. This pathway represents a metabolic adaptation that may protect the tissues from the catabolic effects of thyroid hormone that could otherwise worsen the patient’s basic clinical condition.

Euthyroid sick syndrome or hypothyroid?

In a variety of systemic illnesses, some patients with low T₃, low or normal T₄, and normal thyroid-stimulating hormone (TSH) levels could in fact be “sick euthyroid” rather than hypothyroid. The first reports of the euthyroid sick syndrome or low T₃ syndrome date back to about 1976, and even though assays for reverse T₃ were not widely available, some authors linked the syndrome to high levels of reverse T₃.^15,16 The syndrome is also known as nonthyroidal illness syndrome.

Advances in techniques for measuring T₃, reverse T₃, and other iodothyronines filled a gap in the understanding of the alterations that occur in thyroid hormone economy during severe nonthyroidal diseases. In 1982, Wartofsky and Burman²⁵ reviewed the alterations in thyroid function in patients with systemic illness and discussed other factors that may alter thyroid economy, such as age, stress, and diverse drugs.

More recently, the low-T₃ syndrome was revisited with a generalized concept regarding the role of D3 in the syndrome.²⁶ D3, normally undetectable in mature tissues, is reactivated in diverse cell types in response to injury and is responsible for a fall in serum T₃ levels. Hypoxia induces D3 activity and mRNA in vitro and in vivo.²⁷ Recent studies have focused on the role of cytokines in the low T₃ syndrome. For instance, interleukin 6 reduces D1 and D2 activity and increases D3 activity in vitro.²⁸

In the outpatient setting, diverse conditions may affect thyroid hormone homeostasis, compatible with mild or atypical forms of low-T₃ syndrome, including caloric deprivation, heart failure, and human immunodeficiency virus infection.²⁹

POSSIBLE CLINICAL UTILITY OF MEASURING REVERSE T₃

In inpatients

Unfortunately, measuring serum reverse T₃ levels has not, in general, proven clinically useful for the diagnosis of hypothyroidism in systemically ill patients. Burmeister³⁰ demonstrated, in a retrospective study, that when illness complicates the interpretation of thyroid function tests, serum reverse T₃ measurements do not reliably distinguish the hypothyroid sick patient from the euthyroid sick patient. The best way to make the diagnosis, Burmeister suggested, is by clinical assessment, combined use of free T₄ and TSH measurements, and patient follow-up.

Indeed, few clinical situations require measurement of reverse T₃ levels. We believe it can potentially be used to help the differential diagnosis between hypothyroidism and euthyroid sick syndrome. Reverse T₃ should always be analyzed in combination with TSH, T₃, and free T₄ with consideration of the patient’s clinical context. Table 1 helps to interpret the results. However, even in these circumstances, serum reverse T₃ levels are not always reliable, as demonstrated by Burmeister.³⁰ Another situation, even rarer, is in children or adults with massive hemangiomas. These tumors express high levels of D3 that can cause hypothyroidism.³¹

In the outpatient setting, the utility of reverse T3 measurements is controversial. In intensive care units, the differential diagnosis between hypothyroidism and nonthyroidal illness syndrome can sometimes be difficult. Reverse T₃ levels can be low, normal, or high regardless of the thyroidal state of the patient.³⁰ Moreover, endogenous changes in the hypothalamic-pituitary-thyroid axis may be further complicated by medications commonly used in intensive care units, such as dopamine and glucocorticoids. Changes in thyroid function should be evaluated in the context of the patient’s clinical condition (Table 1).²⁰ But regardless of the T₃ level, treatment with T₃ or T₄ should not be started without taking into consideration the patient’s general clinical context; controlled trials have not shown such therapy to be beneficial.²⁰

In outpatients

In noncritical conditions that may be associated with mild forms of low T₃ syndrome, patients generally present with low T₃ concentrations concurrently with low or normal TSH. Not infrequently, however, patients present with a serum reverse T₃ measurement and impute their symptoms of hypothyroidism to “abnormal” reverse T₃ levels, in spite of normal TSH levels.

There is no rationale for measuring reverse T₃ to initiate or to adjust levothyroxine therapy—the single test relevant for these purposes is the TSH measurement. The risks of basing treatment decisions on reverse T₃ levels include the use of excessive doses of levothyroxine that may lead to a state of subclinical or even clinical hyperthyroidism.

TAKE-HOME MESSAGE

The existence of an inactivating pathway of thyroid hormones represents a homeostatic mechanism, and in selected circumstances measuring serum reverse T₃ may be useful, such as in euthyroid sick patients. The discovery of the molecular mechanisms that lead to the reactivation of D3 in illness is an important field of research.