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Review: Molecular Thyroidology Annals of Clinical & Laboratory Science, vol. 31, no. 3, 2001 221 Review: Molecular Thyroidology William E. Winter and Maria Rita Signorino Department of Pathology, Immunology and Laboratory Medicine, University of Florida College of Medicine, Gainesville, Florida Abstract. Novel disorders involving aberrations of the hypothalamic-pituitary-thyroid gland-thyroid hormone axis have been described in the last 5 to 10 years. The following topics are addressed: molecular mutations causing central hypothyroidism (isolated autosomal recessive TRH deficiency; autosomal recessive TRH-receptor inactivating mutations; TSH beta-subunit bio-inactivating mutations; Pit-1 mutations; Prop1 mutations; high molecular weight bio-inactive TSH); defects in response to TSH (mutations in the TSH receptor: TSH receptor gain-of-function mutations; TSH receptor loss-of-function mutations); defects in thyroid gland formation: transcription factor mutations (TTF-2 and Pax8); defects in peripheral thyroid hormone metabolism (defective intrapituitary conversion of T4 to T3; hemangioma consumption of thyroid hormone); and defects in tissue response to thyroid hormone (generalized thyroid hormone resistance, selective pituitary thyroid hormone resistance). While molecular diagnosis of such conditions is rarely indicated for clinical management, knowledge of the molecular mechanisms of these diseases can greatly enhance the clinical laboratory scientist’s ability to advise clinicians about appropriate thyroid testing and to interpret the complex and sometimes confusing results of thyroid function tests. (received 17 March 2001; accepted 20 March 2001) Key words: TRH, TRH receptor, TSH, TSH receptor, thyroid hormone receptor Introduction Normal Thyroid Function The goal of this review is to introduce the clinical The hypothalamus and anterior pituitary gland laboratorian to several recent advances in molecular thyrotrophs monitor free thyroid hormone levels in thyroidology. Many novel disorders involving the blood stream (Fig. 1). Unbound or free triiodo- aberrations of the hypothalamic-pituitary-thyroid thyronine (FT3) is present in the plasma and enters gland-thyroid hormone axis have been described in the parvicellular division of the paraventricular nucleus. the last 5 to 10 years. While molecular diagnosis of Within these paired hypothalamic nuclei that are such conditions is rarely indicated for clinical adjacent to the superior aspect of the third ventricle, management, knowledge of the molecular mechanisms intracellular T3 is also derived from monodeiodination of disease can greatly enhance the laboratorian’s ability of free tetraiodothyronine (free thyroxine, FT4) that to advise clinicians about appropriate thyroid testing, has entered the cell cytoplasm from the plasma. If the and to interpret the complex and sometimes confusing intracellular level of T3 declines, thyrotropin-releasing results of thyroid function tests. This review begins hormone (TRH) is released into the hypothalamic- with a brief overview of the normal hypothalamic- pituitary-portal system to be delivered to the anterior pituitary-thyroid gland-thyroid hormone axis. pituitary gland. TRH is the tripeptide pyroGlu-His- Pro-NH2. The cyclized glutamic acid terminus and Address correspondence to William E. Winter, M.D., Department an intact amide are required for TRH bioactivity. TRH of Pathology, Immunology and Laboratory Medicine, University sensitizes the anterior pituitary thyrotrophs to release of Florida Medical School, Box 100275, Gainesville, FL 32610- 0275, USA; tel 352 392 4495; fax 352 846 2149; e-mail more thyroid stimulating hormone (TSH) if intra- [email protected]. cellular thyrotroph T3 levels are deficient. 0091-7370/01/0300/0221 $6.00; © 2001 by the Association of Clinical Scientists, Inc. 222 Annals of Clinical & Laboratory Science TSH circulates systemically. Upon binding of TSH to the TSH receptor located on thyroid follicular cell, many processes are activated to increase the release of thyroid hormone into the circulation. T4 is derived only from the thyroid gland. On the other hand, only about 20% of T3 is directly generated from the thyroid gland, with about 80% of T3 being derived from peripheral monodeiodination of T4 to T3. The majority of thyroid hormone is bound to plasma proteins, including the alpha-1 globulin thyroxine- binding globulin (TBG), thyroxine-binding prealbumin (now called transthyretin), and albumin. Only 0.03% of T4 and 0.3% of T3 are unbound. The unbound or “free” fractions of thyroid hormone are the biologically active forms of thyroid hormone in the circulation. With a rise in plasma FT3 and intracellular T3 in the pituitary (and to a lesser degree in the hypothalamus), TSH and TRH secretion are Fig. 1. The hypothalamus secretes thyrotropin releasing hormone suppressed, completing the negative feedback loop for (TRH) into the hypothalamic-pituitary portal system. In turn, control of thyroid hormone synthesis and secretion. TRH regulates the responsiveness of thyrotropin (TSH) to thyroid Thyroid hormone is “trophic” for many tissues. hormone feedback. TSH circulates systemically and stimulates the Thyroid hormone is important for the growth, thyroid gland to release thyroxine (tetraiodothyronine, T4) and 3,5,3'-triiodothyronine (T3). About 80% of circulating T3 is differentiation, and maintenance of the central nervous derived from peripheral monodeiodination of T4 to T3. In target system (very important), skeleton (very important), tissues, T4 is also converted to T3 and the T3 interacts with nuclear cardiovascular system, and gastrointestinal system. thyroid hormone receptors. Basal metabolic rate (BMR) is directly regulated by thyroid hormone. Thyroid hormone also affects and Thyrotrophs, the anterior pituitary cells that release regulates intermediary metabolism, drug metabolism, TSH in response to TRH and decreased T3, express and the activity of other hormones (eg, growth TRH receptors. When TRH binds to the TRH hormone secretion is impaired in individuals with receptor, the thyrotroph depolarizes, allowing calcium hypothyroidism). to influx into the thyrotroph cytoplasm. In turn, increased free cytosolic calcium activates the Ca2+- Overview of Molecular Thyroidology phosphatidylinositol cascade. This causes TSH release and synthesis and glycosylation of alpha and beta TSH “New explanations for old diseases” would be an subunits. Stimulation of glycosylation of TSH subunits appropriate title for much of this review. The following is relatively a greater effect of TRH than stimulation topics will be addressed: of TSH synthesis. Glycosylation is necessary for Molecular mutations causing central hypothyroidism: bioactivity of TSH. TRH also depresses T3 receptor • Isolated autosomal recessive TRH deficiency, expression. This makes the thyrotroph less sensitive to • Autosomal recessive TRH-receptor inactivating mutations, thyroid hormone negative feedback, further increasing • Pit-1 mutations, TSH release. The major site of central negative • PROP-1 mutations; feedback is the pituitary. However, injected TRH TSH beta-subunit bio-inactivating mutations: normally releases TSH and prolactin. The lactotrophs • High molecular weight bio-inactive TSH; express the TRH receptor. For all other hormones Defects in response to TSH: mutations in the TSH receptor: regulated negatively by the hypothalamus and pituitary, • TSH receptor gain-of-function mutations, the major site of negative feedback is the hypothalamus. • TSH receptor loss-of-function mutations; Review of molecular thyroidology 223 Defects in thyroid gland formation: transcription factor degree of sialylation increases (an effect of estrogen), mutations: the half-life of TBG increases, raising TBG levels. In • TTF-2, Pax8 [1]; congenital TBG excess, an X-linked dominant Defects in peripheral thyroid hormone metabolism: condition, male hemizygotes display 3- to 5-fold • Defective intrapituitary conversion of T4 to T3 [2], elevations in TBG levels, while female heterozygotes • Hemangioma consumption of thyroid hormone; Defects in response of tissues to thyroid hormone: display 2- to 3-fold increases in TBG. Besides estrogen • Generalized thyroid hormone resistance [3], effects and congenital excess, other causes of elevated • Selective pituitary thyroid hormone resistance. TBG levels include acute liver disease and drugs (eg, phenothiazines). While many defects in thyroid hormone In familial dysalbuminemic hyperthyroxinemia, a biosynthesis have been described, their etiology is mutant dominantly-inherited form of albumin generally understood and they are not reviewed in detail (Arg218His) binds increased amounts of T4 but not in this paper. However,the clinical laboratory scientist T3, producing euthyroid hyperthyroxinemia, with should be familiar with cases of goitrous congenital normal T3 levels as well as normal FT4 levels. In hypothyroidism and non-autoimmune goitrous contrast, TBG excess raises both T4 and T3. Another hypothyroidism that result from: cause of euthyroid hyperthyroxinemia is familial • defects in iodine transport into the thyroid gland euthyroid thyroxine excess that results from a mutant (autosomal recessive mutations in the sodium/iodide form of transthyretin (TTR, Thr119Met). An older symporter located on chromosome 19p12-13.2) [4], name for TTR is thyroxine-binding prealbumin • organification defects (autosomal recessive (TBPA). Encoded by the TTR gene on chromosome mutations of thyroperoxidase located on chromosome 18q11.2, TTR exists as
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