Thyroid Hormones (T4, T3)

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From: Endocrinology: Basic and Clinical Principles, Second Edition (S. Melmed and P. M. Conn, eds.) © Humana Press Inc., Totowa, NJ
17 Thyroid Hormones (T4, T3)
Takahiko Kogai, MD, PhD and Gregory A. Brent, MD
CONTENTS
INTRODUCTION
THYROID HORMONE–BINDING PROTEINS AND MEASUREMENT OF THYROID HORMONE LEVELS
MOLECULAR ACTION OF THYROID HORMONE
CLINICAL MANIFESTATIONS OF REDUCED THYROID HORMONE LEVELS
CLINICAL MANIFESTATIONS OF EXCESS THYROID HORMONE LEVELS
THYROID HORMONE RESISTANCE
mone despite variation in the supply of dietary iodine. Thyroid hormone influences a wide range of processes, including amphibian metamorphosis, development, reproduction, growth, and metabolism. The specific processes that are influenced differ among species, tis- sues, and developmental phase.
2. THYROID HORMONE SYNTHESIS The synthesis of thyroid hormones requires iodide,
thyroid peroxidase (TPO), thyroglobulin, and hydro- gen peroxide (H2O2). Iodine is transported into the thyroid in the inorganic form by the sodium/iodide symporter (NIS), oxidized by the TPO-H2O2 system, and then utilized to iodinate tyrosyl residues in thyro- globulin. Coupling of iodinated tyrosyl intermediates in the TPO-H2O2 system produces T4 and T3, which are hydrolyzed and then secreted into the circulation. These processes are closely linked, and defects in any of the components can lead to impairment of thyroid hormone production or secretion.
2.1. Structure of Thyroid Follicle The functional unit for thyroid hormone synthesis
and storage, common to all species, is the thyroid fol-
1. INTRODUCTION Thyroid hormone is produced by all vertebrates. In
mammals, the thyroid gland is derived embryologically from endoderm at the base of the tongue and develops into a bilobed structure lying anterior to the trachea. The structure and arrangement of thyroid tissue, however, vary significantly among species. Several key transcrip- tion factors, thyroid transcription factors 1 and 2 (TTF 1 and 2) and Pax8, are required for normal thyroid gland development and regulate gene expression in the adult thyroid gland. The thyroid gland receives a rich blood supply, as well as sympathetic innervation, and is spe- cialized to synthesize and secrete thyroxine (T4) and triiodothyronine (T3) into the circulation (Fig. 1). This process is regulated by thyroid-stimulating hormone ([TSH], or thyrotropin) secreted from the pituitary, which is, in turn, stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. Both TSH and TRH are regulated in a negative-feedback loop by cir- culating T4 and T3. Iodine and the trace element sele- nium are essential for normal thyroid hormone metabolism. Regulatory mechanisms within the thy- roid gland allow continuous production of thyroid hor-
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licle (Fig. 2). The follicle consists of cells arranged in a spherical structure. The thyroid cell synthesizes thy- roglobulin, which is secreted through the apical mem- brane into the follicle lumen. The secreted substance containing thyroglobulin, colloid, serves as a storage form of iodine and is resorbed to provide substrate for T4 and T3 synthesis. The amount of stored colloid var- ies as a result of a number of conditions, including the level of TSH stimulation and availability of iodine. With TSH stimulation, colloid is resorbed to synthe- size thyroid hormone, and with chronic stimulation, the size of the follicular lumen decreases. TSH also stimulates expression of elements of the cytoskeleton, which mediate changes in follicular cell shape that
favor thyroid hormone production. The organization of thyroid cells in culture from monolayer to follicles stimulates NIS gene expression and iodide uptake. Defects in thyroid hormone synthesis or release can result in increased colloid stores.
2.2. Thyroglobulin Thyroglobulin is the major iodoprotein of the thyroid
gland. It is a large dimeric glycoprotein (660 kDa) that serves as a substrate for efficient coupling of mono- iodotyrosine (MIT) and diiodotyrosine (DIT) by the TPO-H2O2 system, to produce T4 and T3, as well as provides a storage form of easily accessible thyroid hormone (Fig. 3). Because of this storage capacity, the
Fig. 2. Photomicrograph of thyroid follicles of varying sizes. Each follicle consists of a ring of cells filled with colloid.
Fig. 1. Structure of L-thyroxine (T4) and its major metabolites, T3 and reverse T3 (rT3). The enzymes include type I 5´-deiodinase (D1), type II 5´-deiodinase (D2), and type III 5-deiodinase (D3).
Chapter 17 / Thyroid Hormones (T4, T3) 269
thyroid gland can continue to secrete thyroid hormone despite transient deficiencies in environmental iodine. The amount of stored thyroglobulin varies among spe- cies, with rodents having limited stores and humans very large stores, sufficient for up to 1 mo of thyroid hormone production. Thyroglobulin synthesized on the endoplas- mic reticulum is transported to the Golgi apparatus, where carbohydrate moieties are added. The thyroglo- bulin is then localized at the apical membrane, where internal tyrosyl residues are iodinated by TPO and H2O2. Because iodide is bound to an organic compound, this process is known as “organification” of iodide.
The intracellular generation of H2O2 is essential for thyroglobulin iodination and coupling and is generated by the thyroid follicular cell (Fig. 3). TSH stimulates uptake of glucose, which is metabolized by the pentose monophosphate shunt, generating NADPH from NADP. The reduced adenosine nucleotide, NADPH, and NADPH oxidase are considered the major mechanisms for the reduction of molecular oxygen to H2O2.
Coupling between two DIT moieties forms T4, and coupling of MIT and DIT produces T3 (Fig. 3). The coupling reaction is catalyzed by TPO and involves the cleavage of a tyrosyl phenolic ring, which is joined to an iodinated tyrosine by an ether linkage. The structural integrity of the thyroglobulin protein matrix is essential for efficient coupling. The usual thyroidal secretion contains about 80% T4 and 20% T3, however, the ratio of secreted T4:T3 can be altered. Hyperstimulation of the TSH receptor is associated with an increase in the relative fraction of T3 secretion. TSH receptor stimula- tion can be owing to IgG in Graves disease or a consti- tutive activating TSH receptor mutation found in many hyperfunctioning autonomous nodules. The excess in T3 is owing to preferential MIT/DIT coupling as well as increased activity of the intrathyroidal type I (D1) and type II (D2) 5´-deiodinase that converts T4 into T3 (see Section 3). Repletion of iodine after a period of iodine deficiency also results in an increase in the frac- tion of T3 in the thyroidal secretion.
Fig. 3. Diagram of major steps involved in thyroid hormone synthesis and secretion. Tg = thyroglobulin; ECF = extracellular fluid; 5´ D Type I = 5´-iodothyronine deiodinase type I; Na/I symporter = sodium iodide symporter; ATP = adenosine triphosphate; DIT = diiodityrosine; MIT = monoiodotyrosine.
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The process of thyroid hormone release and secretion begins with TSH-stimulated resorption of colloid (Fig. 3). Pseudopods and microvilli are formed at the apical membrane, and pinocytosis of colloid produces mul- tiple colloid droplet vesicles. Lysosomes move from the basal to apical region of the cell and fuse with colloid droplets to form phagolysosomes. Proteolysis of thyro- globulin releases iodothyronines, free iodotyrosines, and free amino acids. T4 and T3 then diffuse across the cell and into the circulation.
2.3. Iodine Transport The thyroid contains 70–80% of the total iodine in
the body (15–20 mg). The thyroid gland must trap about 60 μg of iodine/d from the circulation to main- tain adequate thyroid hormone production. The uri- nary excretion of iodine generally matches intake, and low levels indicate inadequate iodine intake. NIS is a membrane-bound protein located in the basolateral portion of the thyroid follicular cell that passively transports two Na+ and one I- down the Na+ ion gradi- ent, resulting in an iodine concentration gradient from the thyroid cell to extracellular fluid of 100:1 (Fig. 3). The iodide gradient can be increased to as high as 400:1 in conditions of iodine deficiency. Iodine transport is driven by the Na+ gradient generated from Na+/K+
adenosine triphosphatase (ATPase). Ouabain, which inhibits the Na+/K+ ATPase, blocks thyroidal iodide uptake. Iodide uptake by NIS, therefore, is a passive, but efficient, transport process that occurs against an iodide electrochemical gradient. The process is stimu- lated by TSH via cyclic adenosine monophosphate (cAMP). TSH induces NIS gene expression through the thyroid-selective enhancer located far upstream of the gene with cAMP-regulated transcription factors, such as Pax-8 and cAMP-response element–binding protein (CREB). Trapped iodide in the follicular cells is further transferred to the lumen by other iodide trans- porters at the apical membrane, pendrin or the apical iodide transporter (AIT), and “organified” with thyro- globulin for subsequent thyroid hormone synthesis. Io- dine transport by NIS is seen in other tissues, including the salivary gland, gastric mucosa, lactating mammary gland, ciliary body of the eye, and the choroid plexus. A low level of iodide uptake has been demonstrated in breast cancer. Iodine is not organified in these tissues, other than lactating mammary glands, and NIS gene expression is unresponsive to TSH.
Endemic goiter is the presence of thyroid enlarge- ment in >10% of a population, a higher fraction than that owing to intrinsic thyroid disease alone, and indi- cates that the etiology is likely to be owing to dietary and/or environmental factors. Most endemic goiters
are the result of reduced thyroidal iodine resulting from deficient dietary iodine. Mountainous areas, including the Andes and Himalayas, as well as central Africa and portions of Europe, remain relatively iodine deficient. Reduced thyroidal iodine may also be the result of fac- tors that inhibit NIS. Inhibitors can be natural dietary “goitrogens,” such as the cyanogenic glucosides found in cassava, a staple in parts of Africa and Asia. Cyano- genic glucosides are hydrolyzed in the gut by glucosi- dases to free cyanide, which is then converted into thiocyanate. Thiocyanate inhibits thyroid iodide trans- port and at high concentrations interferes with organi- fication. Other inhibitors include perchlorate, chlor- ate, periodate, and even high concentrations of iodide, which cause transient inhibition of thyroid hormone synthesis (Wolff-Chaikoff effect). This has been shown to be primarily owing to reduced NIS expres- sion. Perchlorate causes release of nonorganified io- dine and is used diagnostically, after radioiodine tracer uptake, to distinguish defects of iodine uptake from organification (radioiodine transported but not organified will be released after administration of per- chlorate). Perchlorate is used in the aircraft and rocket industry and has been detected in various concentra- tions in water supplies worldwide. The impact of vari- ous levels of perchlorate on thyroid function in adults and children is being studied. A wide range of heritable defects also result in impaired iodide transport or organification, including genetic mutations of iodide transporters, NIS and PDS, which encodes pendrin. Mutation of PDS in patients with Pendred syndrome leads to inefficient iodide transport to the follicular lumen and brings about a “partial” organification de- fect in the thyroid. In this case, trapped radioiodine in the thyroid can be discharged by perchlorate faster than normal.
NIS is utilized clinically for both diagnostic and therapeutic applications. Radioisotopes of iodine can be given orally and are taken up into thyroid tissue with high efficiency. Nonincorporated iodine is rapidly excreted by the kidneys. Short-half-life, low-energy iso- topes, such as I123, are used to make images of functional thyroid tissue. Longer-half-life, high-energy isotopes, such as I131, are used therapeutically to destroy thyroid tissue in both hyperthyroidism and thyroid cancer. Thy- roid cancer requires a high level of TSH stimulation, either endogenous after thyroidectomy and cessation of thyroid supplementation, or exogenous administration of recombinant TSH. Less-differentiated thyroid can- cers, however, either do not have or lose the ability to transport iodine. Agents that target stimulation of NIS expression or augmentation of its function in these situ- ations are being developed as therapeutic tools.
Chapter 17 / Thyroid Hormones (T4, T3) 271
2.4. Thyroid Peroxidase TPO is a membrane-bound glycoprotein with a cen-
tral role in thyroid hormone synthesis catalyzing iodine oxidation, iodination of tyrosine residues, and iodothy- ronine coupling. The human cDNA codes for a 933- amino-acid protein with transmembrane domains at the carboxy terminus. The extracellular region contains five potential glycosylation sites. The human thyroid per- oxidase gene is found on chromosome 2 and spans approx 150 kb with 17 exons. The 5´-flanking sequence contains binding sites for a number of thyroid-specific transcription factors, including TTF 1 and 2. TSH stimulates TPO gene expression by an increase in intra- cellular cAMP, although the level of regulation (tran- scriptional vs posttranscriptional) varies by species.
IgG autoantibodies to TPO are pathogenic in several thyroid diseases. The predisposition to forming TPO autoantibodies is inherited as an autosomal-dominant trait in women but has incomplete penetrance in men. This pattern of inheritance is consistent with the female preponderance of autoimmune thyroid disease. In addi- tion to the diagnosis of autoimmune thyroid disease, the magnitude of elevation of these antibodies correlates with disease activity. TPO antibodies are known to dam- age cells directly by activating the complement cascade. A number of epitopes for TPO autoantibodies have been defined. Several animal models with thyroid autoanti- bodies have demonstrated that a second insult, such as injection of interferon or other cytokine, is required for thyroid destruction and hypothyroidism. Clinically, thy- roid destruction can be transient, with temporary phases of increased and then decreased thyroid hormone levels (lymphocytic thyroiditis) or permanent hypothyroidism (Hashimoto disease). Lymphocytic thyroiditis is often seen in the postpartum period.
2.5. Influence of Thyrotropin on Thyroid Hormone Synthesis
The major stimulus to thyroid hormone production and thyroid growth is stimulation of the TSH receptor. Other factors that modify this response include neu- rotransmitters, cytokines, and growth factors. In addi- tion to physiologic regulation via TSH, there are a number of clinical disorders of excess and reduced thy- roid hormone production mediated by the TSH receptor.
The human TSH receptor gene is on the long arm of chromosome 14 and consists of 10 exons spread over 60 kb. Analysis of the regulatory region of the gene has identified binding sites for TTF 1 and 2, as well as cAMP response elements. TSH is a G protein–coupled recep- tor with a classic seven-transmembrane domain struc- ture. The primary structure contains leucine-rich motifs and six potential N-glycosylation sites. Such motifs are
similar to those that form amphipathic α-helices and may be involved in protein-protein interactions. A num- ber of recent studies have demonstrated that full func- tion of the TSH receptor results from cleavage of a portion of the extracellular domain. This appears to be a unique feature of the TSH receptor, and antibodies to the cleaved portion may play an important role in the pathogenesis of Graves disease and especially extrathyroidal manifestations. The receptors for the pituitary glycoprotein hormones—TSH, follicle-stimu- lating hormone (FSH), and leutinizing hormone (LH)/ chorionic gonadotropin (CG)—are very similar in the transmembrane domain containing the carboxy-termi- nal portion (70%) but have less similarity in the extra- cellular domain (about 40%). The similarity is clinically relevant in glycoprotein hormone “spillover” syn- dromes, in which marked elevations in these hormones stimulate related receptors. Excess CG from tropho- blastic disease can stimulate thyroid hormone produc- tion via the TSH receptor, and excess TSH in pre- pubertal children with primary hypothyroidism can stimulate precocious puberty via stimulation of the FSH and/or LH/CG receptors. A TSH receptor mutation that retained normal TSH affinity, but a marked augmenta- tion of hCG affinity has been reported. Affected indi- viduals were thyrotoxic only during pregnancy. Gain-of-function mutations have been identified in the TSH receptor, resulting in constitutive activation (TSH independent) of thyroid hormone production. These mutations are manifest in the heterozygous state, pro- duce thyroid growth as well as an increase in thyroid function, and have been found in the majority of hyperfunctioning thyroid nodules. Similar constitutive mutations in the germ line produce diffuse thyroid hyperfunction and growth. Inactivating TSH receptor gene mutations have also been reported. Characteriza- tion of these mutations has helped to map functional domains of the TSH receptor.
TSH stimulation of thyroid follicular cells pro- motes protein iodination, thyroid hormone synthesis, and secretion. These effects can be reproduced by agents that enhance cAMP accumulation (theophyl- line, cholera toxin, forskolin, cAMP analogs). At high concentrations of TSH, there is activation of the Ca2+
phosphatidylinositol-4,5-bisphosphate (PIP2) cascade. The relative influence of the cAMP and PIP2 pathways appears to differ by species; for example, dog have only the cAMP pathway and humans have both. TSH acting via cAMP generation stimulates the expression of a number of genes involved in thyroid hormone syn- thesis and secretion, including NIS, thyroglobulin, and TPO. In many species (e.g., human, rat, and dog), TSH is mitogenic and promotes thyroid growth.
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2.6. Interference of Antithyroid Drugs With Thyroid Hormone Synthesis
In the 1940s, the thionamides were first observed to produce goiters in laboratory animals. Propylthiouracil and methimazole are the most commonly used of these compounds, and both have intrathyroidal and extrathy- roidal actions. They reduce thyroid hormone production by interfering with the actions of TPO, which include the oxidation and organification of iodine, and the cou- pling of MIT and DIT to form T4 and T3. The thion- amides compete with thyroglobulin tyrosyl residues for oxidized iodine. These medications are primarily used in patients with hyperthyroidism resulting from Graves disease but are effective in any form of hyperthyroidism owing to overproduction of thyroid hormone. Propylth- iouracil has an additional effect at high serum T4 con- centrations of reducing peripheral T4 to T3 conversion by inhibiting the D1. Both agents are thought to have additional immunosuppresive actions that may help in the treatment of autoimmune hyperthyroidism.
3. THYROID HORMONE METABOLISM The thyroid gland secretes primarily T4, which must
be converted into the active form, T3, by D1 (Fig. 1). The various pathways of thyroid hormone metabolism allow regulation of hormone activation at the target tissue level as well as adaptation for times of reduced thyroid hor- mone production. A large number of iodothyronine metabolites are degradation products of T4, in addition to T3, including rT3 (3,3´,5´-triiodothyronine), T2S, 3´- T1, and T0. The levels of these products vary in a number of thyroid states and, in some situations, have been used diagnostically. Reverse T3 e.g., is metabolically inac- tive but is elevated in illness and fasting. The liver solu- bilizes T4 metabolites by sulfation or glucuronide
formation for excretion by the kidney or in the bile. The process allows the conservation of body iodine stores.
Deiodinase enzymes have distinctive characteristics based on developmental expression; tissue distribution; substrate preference; kinetics; and sensitivity to inhibi- tors, such as propylthiouracil and iopanoic acid. Deiodinases can be separated into phenolic (outer ring) 5´-deiodinases or tyrosyl (inner ring) 5-deiodinases (Table 1, Fig. 1).
3.1. Type I 5´-Deiodinase (D1) The primary source of T3 in the peripheral tissues is
D1, although rodents and humans differ in the contribu- tion of D1 (Fig. 4). This enzyme is found predominantly in thyroid, liver, and kidney. T3, TSH, and cAMP all increase expression of D1 in FRTL5 thyroid cell cul- tures. Consistent with this observation are the in vivo findings of increased D1 activity in hyperthyroidism and reduced activity in hypothyroidism. The biochemi- cal properties of D1 include a preference for rT3 as a substrate over T4. D1 requires reduced thiol as a cofac- tor and is sensitive to inhibition by propylthiouracil and gold. Other inhibitors of D1 include illness, starvation, glucocorticoids, and propranolol.
3.2. Type II 5´-Deiodinase (D2) D2 is a related 5´-deiodinase with distinct tissue dis-
tribution, biochemical properties, and physiologic func- tion. D2 is found primarily in the pituitary, brain, muscle, and brown fat. This enzyme functions to regulate intra- cellular T3 levels in tissue, where an adequate concen- tration is critical. In humans, D2 may be the major contributor to T3 production (Fig. 4). The biochemical properties include a preference for T4 over rT3 as a sub- strate and insensitivity to inhibition by propylthiouracil. The activity of D2 increases in hypothyroidism, appar-
Table 1 Properties of Iodothyronine Deiodinases
D1 D2 D3
Developmental expression Expressed in…