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Thyroxine-binding globulin

Thyroxine-binding globulin (TBG), encoded by the SERPINA7 gene, is a 54 kDa glycoprotein synthesized primarily in the liver that functions as the major transport protein for thyroid hormones in human serum.^[1] It reversibly binds approximately 75% of circulating thyroxine (T4) and 70% of triiodothyronine (T3), maintaining a stable pool of these hormones and preventing rapid fluctuations in free hormone levels available to tissues.^[2] With a molecular mass of about 54 kDa and consisting of 395 amino acids, TBG exhibits a higher affinity for T4 than for T3, binding it about 10-fold more strongly, and is only partially saturated (around 25%) under euthyroid conditions.^[3] As a member of the serpin (serine protease inhibitor) superfamily, TBG's structure includes a reactive center loop that facilitates hormone binding, though it does not exhibit protease inhibitory activity in this context.^[1] The SERPINA7 gene is located on the long arm of the X chromosome at Xq22.3 and spans four exons, with expression following an X-linked inheritance pattern that results in hemizygous expression in males and variable levels in heterozygous females due to X-chromosome inactivation.^[3] TBG levels are tightly regulated by physiological factors, including increased synthesis during pregnancy, estrogen therapy, or oral contraceptive use (elevating levels up to 5.5 mg/dL), and decreased levels in response to androgens, hyperthyroidism, or severe nonthyroidal illnesses.^[2] Normal serum TBG concentrations range from 1.2 to 2.5 mg/dL in adult males and vary by age and sex, with newborns showing higher levels that decline postnatally.^[2] Clinically, variations in TBG are significant for interpreting thyroid function tests, as inherited deficiencies—either complete (prevalence 1:15,000 male newborns) or partial (1:4,000 male newborns)—do not cause thyroid dysfunction but can mimic hypothyroidism or hyperthyroidism due to low total T4/T3 levels despite normal free hormone concentrations.^[1] Over 40 mutations in SERPINA7 have been identified, including missense variants that reduce binding affinity (e.g., TBG-Gary, Ile96Asn) and frameshift mutations leading to complete deficiency, all of which are X-linked without metabolic consequences.^[3] Acquired alterations in TBG, such as those from liver disease or medications, further underscore its role in thyroid hormone homeostasis, emphasizing the need for direct measurement of free thyroid hormones in diagnosis.^[1]

Molecular Biology

Gene

Thyroxine-binding globulin (TBG) is encoded by the SERPINA7 gene, located on the long arm of the X chromosome at cytogenetic band Xq22.2.^[4] This gene spans approximately 6.3 kilobases and consists of five exons, including a short noncoding first exon and four coding exons that produce a mature mRNA encoding a 415-amino-acid precursor protein.^[5] As a member of the serpin (serine protease inhibitor) superfamily, SERPINA7 shares structural and functional homology with other serpins, such as alpha-1-antitrypsin encoded by SERPINA1, reflecting evolutionary conservation across the family in roles involving proteinase inhibition and ligand binding.^[4] The inheritance of SERPINA7 variants follows an X-linked pattern, with males being hemizygous and females potentially heterozygous, leading to variable expression due to X-chromosome inactivation.^[3] Complete TBG deficiency typically arises from null mutations in SERPINA7, such as nonsense mutations (e.g., c.158C>T, p.Gln53*) or frameshift variants that introduce premature stop codons, resulting in absent or nonfunctional protein.^[3] In contrast, partial TBG deficiency is often caused by missense mutations that impair protein stability or function without abolishing expression, with examples including p.Pro363Leu, a recurrent variant associated with reduced TBG levels in affected individuals.^[3] These genetic alterations underscore the gene's critical role in thyroid hormone transport, though detailed binding mechanisms are addressed elsewhere.^[3]

Protein Structure

Thyroxine-binding globulin (TBG) is a glycoprotein with a molecular mass of approximately 54 kDa, comprising a single polypeptide chain of 395 amino acids and accounting for about 20% of its mass as carbohydrates.^[6] The mature protein is derived from a precursor of 415 amino acids after cleavage of a 20-residue signal peptide, and it belongs to the serpin (serine protease inhibitor) superfamily, adopting the characteristic serpin fold with three β-sheets and nine α-helices.^[6]^[7] Unlike typical inhibitory serpins, TBG lacks protease inhibitory activity and is instead specialized for ligand transport, utilizing its reactive center loop (RCL)—a flexible segment above the central β-sheet—for high-affinity binding of thyroid hormones.^[6] The RCL in TBG features a unique proline residue at the P8 position, which restricts full insertion into the β-sheet upon cleavage, enabling reversible conformational changes that modulate hormone release without permanent structural alteration.^[8] TBG contains four N-linked glycosylation sites at asparagine residues (Asn-31, Asn-194, Asn-232, and Asn-402 in the precursor numbering), which are essential for proper protein folding, secretion, and circulatory half-life, as the terminal sialic acids prevent rapid hepatic clearance.^[6]^[7] These sites attach complex heterosaccharide chains, contributing to the protein's acidic nature and stability in plasma.^[9] TBG exhibits high binding affinity for thyroxine (T4), with an association constant (Ka) of approximately 10^{10} M^{-1}, which is about tenfold greater than for triiodothyronine (T3) (Ka ≈ 10^{9} M^{-1}), allowing preferential transport of T4 in the bloodstream.^[6] Crystal structures of TBG, resolved at resolutions up to 1.55 Å, reveal that the hormone-binding pocket is a shallow surface cavity located between α-helix H and the adjacent α-helix A, as well as strands 3–5 of the B β-sheet, where T4 is stabilized by hydrophobic interactions and hydrogen bonds involving key residues like Arg^{228}, His^{247}, and Tyr^{176}.^[8] Upon limited proteolysis, partial insertion of the RCL into the β-sheet contracts this pocket, reducing affinity and facilitating hormone delivery to tissues.^[8]

Physiological Role

Synthesis and Secretion

Thyroxine-binding globulin (TBG) is primarily synthesized in the hepatocytes of the liver, where it is produced as a glycoprotein encoded by the SERPINA7 gene. Although the liver accounts for the majority of TBG production, minor expression of SERPINA7 has been detected in other tissues, including the kidney cortex, albeit at much lower levels compared to hepatic tissue. This hepatic synthesis is essential for maintaining circulating TBG levels, which serve as the primary carrier for thyroid hormones in plasma.^[6]^[10]^[11] The secretion of TBG involves classical endoplasmic reticulum-Golgi processing pathways typical of secreted glycoproteins. Newly synthesized TBG is translocated into the endoplasmic reticulum, where core N-linked glycosylation occurs at four sites, initiating proper folding and quality control. The protein then traffics to the Golgi apparatus for further modifications, including terminal glycosylation and sialylation, before packaging into secretory vesicles for exocytosis into the bloodstream. These post-translational modifications, particularly glycosylation, are critical for TBG stability and secretion efficiency, as disruptions can impair release from hepatocytes.^[12]^[13] In healthy adults, the daily production rate of TBG is approximately 15-20 mg/day, which sustains plasma concentrations of 12-20 mg/L (or 1.2-2.0 mg/dL). The plasma half-life of TBG is about 5 days, modulated by the extent of sialylation on its glycan chains, with more heavily sialylated forms exhibiting reduced clearance by hepatic asialoglycoprotein receptors. Ontogenetically, TBG synthesis commences in the fetal liver during the first trimester, with detectable levels in fetal serum as early as 12 weeks of gestation; newborn levels are elevated (about 1.5 times adult values), gradually declining to adult concentrations by 2-3 years of age.^[14]^[15]^[1]

Hormone Binding

Thyroxine-binding globulin (TBG) serves as the primary carrier for thyroid hormones in the bloodstream, binding approximately 75% of circulating thyroxine (T4) and 70% of triiodothyronine (T3).^[16] This binding occurs through a single high-affinity site on each TBG molecule, with a notably higher affinity for T4 compared to T3, approximately 10-fold greater, as reflected in association constants (Ka) of 1 × 10¹⁰ M⁻¹ for T4 and 1 × 10⁹ M⁻¹ for T3.^[6] The equilibrium dissociation constant (Kd) for T4 is around 0.1 nM, indicating tight binding under physiological conditions.^[6] These interactions are reversible and non-covalent, primarily mediated by a hydrophobic pocket formed by specific structural elements, including helices and beta strands, which accommodate the iodothyronine rings of the hormones.^[8] The binding of thyroid hormones to TBG plays a crucial role in their transport, maintaining a large extrathyroidal pool that buffers against fluctuations in thyroid secretion and protects the hormones from rapid degradation and renal clearance.^[6] By sequestering the majority of T4 and T3, TBG modulates the availability of free, unbound hormone fractions—typically less than 0.03% for T4 and 0.3% for T3—which are the biologically active forms capable of diffusing into target tissues.^[6] This transport mechanism ensures steady delivery to peripheral tissues while preventing excessive loss of iodine, a key component of thyroid hormone synthesis.^[6] Compared to other serum carriers, TBG exhibits the highest affinity for thyroid hormones but the lowest binding capacity due to its relatively low serum concentration (180–350 nM).^[6] Transthyretin (TTR), with a Ka of 2 × 10⁸ M⁻¹ for T4, binds about 20% of circulating T4 and has moderate capacity, while albumin (human serum albumin, HSA), with even lower affinity (Ka ≈ 1.5 × 10⁶ M⁻¹), accounts for roughly 5% but offers the highest capacity owing to its abundance.^[6] In states of elevated hormone levels, such as hyperthyroidism, TBG binding sites become saturated, leading to a proportional increase in the free hormone fractions and potentially altering hormone bioavailability.^[6]

Regulation

Factors Increasing TBG

Estrogen plays a central role in elevating thyroxine-binding globulin (TBG) levels through its influence on hepatic synthesis. During pregnancy, serum TBG concentrations rise progressively, achieving a two- to threefold increase by the third trimester due to elevated estrogen levels stimulating TBG production in the liver.^[17] Similarly, estrogen-containing oral contraceptives induce a comparable elevation in TBG, often within weeks of initiation, by enhancing hepatic glycoprotein synthesis and reducing TBG clearance via increased sialylation.^[18] Hormone replacement therapy in postmenopausal women also boosts TBG levels through analogous estrogen-mediated mechanisms, leading to higher total thyroxine binding capacity.^[2] Genetic factors can cause familial TBG excess, characterized by persistently elevated serum TBG concentrations. This condition arises primarily from duplications or triplications of the SERPINA7 gene on the X chromosome, resulting in increased TBG synthesis rather than altered protein stability from point mutations.^[7] Affected individuals typically exhibit TBG levels 2- to 3-fold above normal, inherited in an X-linked manner without clinical thyroid dysfunction.^[3] Acquired elevations in TBG occur in various liver disorders due to hepatocyte stimulation. In acute hepatitis, serum TBG levels increase significantly, reflecting enhanced synthesis in regenerating hepatocytes rather than passive leakage from damaged cells.^[19] Similarly, chronic liver conditions such as hepatoma are associated with raised TBG, attributable to ongoing hepatic inflammation and altered protein metabolism.^[20] Certain pharmacological agents mimic estrogen effects to raise TBG levels. Tamoxifen, a selective estrogen receptor modulator used in breast cancer treatment, elevates TBG by binding to estrogen receptors in the liver, leading to increased total thyroxine and triiodothyronine binding.^[21] Heroin and methadone similarly augment TBG concentrations through estrogen-like activity, often observed in chronic users with resultant shifts in thyroid hormone profiles.^[22] These increases in TBG generally manifest rapidly following exposure to inducers, with detectable rises occurring within days to weeks. For instance, estrogen administration prompts transcriptional upregulation of the SERPINA7 gene in hepatocytes, enhancing TBG mRNA expression and protein output.^[17] This temporal pattern aligns with TBG's baseline hepatic synthesis, where regulatory factors modulate production without altering the core secretory pathway.^[23]

Factors Decreasing TBG

Thyroxine-binding globulin (TBG) levels can be decreased by various physiological, genetic, acquired, pharmacological, and acute factors, leading to reduced binding capacity for thyroid hormones and potentially affecting total thyroid hormone measurements without altering free hormone levels.^[1] Androgen-related factors contribute to lower TBG concentrations, particularly during male puberty and with androgen therapy. In males, pubertal increases in androgens lead to a significant reduction in TBG levels, which parallels the observed decline in total thyroxine (T4) during this period. Androgen therapy, such as with anabolic steroids or testosterone, can decrease TBG by approximately 50% through mechanisms involving reduced synthesis or increased turnover in the liver.^[24]^[25]^[15] Inherited genetic deficiencies represent a primary cause of low TBG, following an X-linked recessive pattern due to mutations in the SERPINA7 gene on the X chromosome. Complete TBG deficiency results in undetectable TBG levels, while partial deficiency leads to moderately reduced concentrations, affecting approximately 1 in 15,000 to 4,000 male newborns depending on the variant; these conditions are typically asymptomatic but require differentiation from thyroid disorders.^[1]^[2] Acquired conditions that impair TBG synthesis, increase clearance, or cause loss also lower levels. Severe liver disease reduces TBG production since it is synthesized in hepatocytes, leading to decreased circulating concentrations. Nephrotic syndrome causes urinary loss of TBG as a low-molecular-weight protein, resulting in hypoproteinemia and reduced TBG. Hyperthyroidism accelerates TBG clearance and metabolism, contributing to lower levels independent of synthesis changes.^[1]^[26]^[27] Pharmacological agents further suppress TBG through specific inhibitory effects. High-dose glucocorticoids, such as prednisone, decrease TBG by downregulating its hepatic synthesis, an effect observed in conditions like Cushing syndrome or chronic therapy. L-asparaginase, used in leukemia treatment, inhibits TBG glycosylation and biosynthesis, leading to acute reductions in TBG and total thyroid hormone levels.^[28]^[15] Acute stressors, including major surgery or trauma, can transiently decrease TBG levels as part of the systemic response involving interleukin-6 and other cytokines, though this typically resolves with recovery.^[2]

Clinical Significance

Normal Levels and Measurement

In adults, normal serum concentrations of thyroxine-binding globulin (TBG) typically range from 10 to 30 mg/L when measured by immunoassay, with levels often slightly higher in females compared to males due to estrogen influences.^[29]^[30] Reference intervals vary by laboratory and population, but values outside this range in non-pregnant adults may indicate assay-specific differences or require clinical correlation.^[31] TBG is quantified using immunological methods, including radioimmunoassay (RIA), enzyme-linked immunosorbent assay (ELISA), and immunoturbidimetry, which detect the protein through antibody-antigen interactions.^[32]^[33]^[34] These techniques offer high sensitivity for serum samples, with modern variants like immunochemiluminometric assay providing rapid results and low variability.^[29] Levels are commonly reported in μg/mL, equivalent to mg/L, and TBG concentration directly correlates with the total T4 binding capacity, as approximately 70% of circulating thyroxine is bound to TBG.^[31]^[35] Reference intervals for TBG are adjusted for age and physiological states; in neonates, levels are elevated at birth (mean approximately 28 mg/L in the first 4 weeks, with ranges up to 64 mg/L) and decrease progressively toward adult values by adolescence.^[36] During pregnancy, TBG concentrations rise due to estrogen stimulation, reaching 25 to 60 mg/L across trimesters, with the highest values often in the second and third trimesters (e.g., 23-49 mg/L).^[37] Quality control in TBG assays must address potential interferences, particularly from heterophilic antibodies, which can cause falsely elevated or reduced readings by bridging assay antibodies in sandwich formats.^[38] Such interferences occur in 0.05% to 6% of cases depending on the immunoassay platform and patient population, necessitating confirmatory testing with alternative methods if discrepancies arise.^[39]

TBG Abnormalities

Thyroxine-binding globulin (TBG) abnormalities encompass inherited and acquired variations in TBG levels that do not typically cause thyroid dysfunction but can affect measurements of total thyroid hormone concentrations. Inherited forms follow an X-linked pattern of inheritance due to mutations or structural changes in the SERPINA7 gene, which encodes TBG.^[40] Familial TBG excess results from duplications or triplications of the SERPINA7 gene, leading to 2- to 4-fold increases in serum TBG concentrations in affected hemizygous males and heterozygous females.^[40] This condition has an estimated prevalence of approximately 1 in 25,000 individuals.^[40] Affected individuals remain clinically euthyroid with no symptoms, but total thyroxine (T4) and triiodothyronine (T3) levels are elevated while free hormone levels remain normal.^[40] Acquired TBG excess, such as that induced by estrogen therapy, similarly elevates TBG levels and total thyroid hormones without altering free hormone concentrations or causing symptoms.^[2] TBG deficiency occurs in complete and partial forms, both X-linked and resulting in reduced or absent TBG without impacting thyroid function.^[40] Complete TBG deficiency features undetectable serum TBG levels (<0.9 nmol/L) and affects about 1 in 15,000 newborn males, while partial deficiency involves variably reduced TBG concentrations (often 20-50% of normal) with a prevalence of approximately 1 in 4,000 newborns overall.^[40] In both types, total T4 and T3 levels are low, but free hormone levels and thyroid-stimulating hormone are normal, maintaining euthyroid status.^[40] These conditions are generally asymptomatic, though extremes may rarely associate with mild thyroid-related issues due to interpretive challenges in hormone assays.^[1] Partial deficiency shows higher prevalence in certain populations, such as Japanese individuals harboring specific SERPINA7 variants.^[40] Diagnosis of TBG abnormalities relies on genetic testing to identify SERPINA7 variants, often prompted by discordant thyroid function tests showing abnormal total but normal free hormone levels.^[40] Family screening is recommended for confirmation and to identify carriers, as these isolated TBG defects require no treatment beyond avoiding misdiagnosis of thyroid disease.^[40]

Role in Thyroid Diagnosis

Variations in thyroxine-binding globulin (TBG) levels profoundly influence the interpretation of thyroid function tests by altering total thyroid hormone concentrations without affecting the biologically active free fractions. Elevated TBG increases serum total thyroxine (T4) and total triiodothyronine (T3) levels due to enhanced binding capacity, while free T4, free T3, and thyroid-stimulating hormone (TSH) remain unchanged, reflecting true euthyroid status.^[41]^[42] Conversely, reduced TBG lowers total T4 and T3 but spares free levels and TSH.^[2] In clinical scenarios such as pregnancy or estrogen therapy, where TBG rises markedly due to estrogen-mediated hepatic synthesis, reliance on total T4 alone can mimic hyperthyroidism and lead to erroneous diagnoses or overtreatment.^[43]^[44] Guidelines recommend using free T4, free T3, or TSH to circumvent these pitfalls and ensure accurate thyroid assessment.^[45] When discrepancies arise between total and free thyroid hormone measurements, direct quantification of TBG levels aids in resolving the etiology, as binding protein alterations are a common confounder.^[46] Reverse T3 measurements are relatively less impacted by TBG fluctuations in routine diagnostic evaluation, providing supplementary insight in select cases.^[47] Historically, prior to the 1980s, dependence on total T4 assays frequently resulted in diagnostic errors amid unrecognized binding protein variations; the advent of sensitive TSH immunoassays shifted paradigms to a TSH-first strategy, minimizing such inaccuracies.^[48]^[49] In TBG deficiency, low total T4 accompanied by normal TSH and free T4 confirms euthyroidism, averting misguided hypothyroidism therapy.^[1]^[2]

References

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