Thyroid Hormone
Thyroid hormones are two iodine-containing, tyrosine-derived diaryl ether hormones—the prohormone thyroxine (3,5,3′,5′-tetraiodothyronine, T4) and the biologically active hormone triiodothyronine (3,5,3′-triiodothyronine, T3)—synthesized and...
Thyroid hormones are two iodine-containing, tyrosine-derived diaryl ether hormones—the prohormone thyroxine (3,5,3′,5′-tetraiodothyronine, T4) and the biologically active hormone triiodothyronine (3,5,3′-triiodothyronine, T3)—synthesized and secreted by the follicular epithelial cells (thyrocytes) of the vertebrate thyroid gland. Unique among vertebrate signaling molecules in requiring the trace halogen iodine (comprising 65% of the molecular weight of T4 and 59% of T3), thyroid hormones combine the biosynthetic pathway of an amino-acid derivative with the hydrophobic plasma transport and intracellular nuclear receptor mechanism characteristic of steroid and retinoid hormones. First isolated by Edward Calvin Kendall in 1914 and synthesized by Charles Robert Harington in 1926–1927 (followed by the discovery of T3 by Rosalind Pitt-Rivers and Jack Gross in 1952), thyroid hormones act on nearly every nucleated tissue.
Physiologically, T3 and T4 govern whole-body basal metabolic rate (BMR), oxygen consumption (VO2), non-shivering thermogenesis, hepatic lipolysis and cholesterol clearance, cardiac chronotropy and inotropy, fetal and neonatal central nervous system myelination, and the transformation of larval amphibians during metamorphosis.
Biosynthesis in the Thyroid Follicle: How Are T4 and T3 Assembled?
The Sodium–Iodide Symporter (NIS), Pendrin, and Thyroglobulin
The functional unit of the thyroid is the spherical thyroid follicle (20–300 μm), lined by cuboidal follicular epithelial cells enclosing proteinaceous colloid (a 2- to 3-month hormone store). At the basolateral membrane, the sodium–iodide symporter (NIS, SLC5A5) couples inward movement of 2 Na+ ions to active uptake of 1 I− ion, concentrating iodide 20- to 50-fold above plasma (~10−7 M); perchlorate (ClO4−) and thiocyanate (SCN−) competitively inhibit NIS. Iodide exits apically into the colloid via pendrin (SLC26A4; mutated in Pendred syndrome) and anoctamin-1 (ANO1), alongside exocytosis of thyroglobulin (Tg)—a 660 kDa homodimeric glycoprotein with ~134–140 tyrosine residues.
Thyroid Peroxidase (TPO) Organification, Coupling, and Secretion
At the apical membrane, heme-containing thyroid peroxidase (TPO) uses hydrogen peroxide (H2O2) generated by dual oxidase 2 (DUOX2) to catalyze: (1) organification, oxidizing I− to iodinate Tg tyrosyl residues into 3-monoiodotyrosine (MIT) and 3,5-diiodothyrosine (DIT); and (2) intramolecular ether coupling, joining two DITs into thyroxine (T4) or one MIT + one DIT into triiodothyronine (T3). Under TSH stimulation, thyrocytes endocytose colloid droplets into lysosomes, where cathepsins cleave Tg to release T4 (~85–100 μg/day) and T3 (~6 μg/day) via MCT8, while DEHAL1 (IYD) recycles iodide from uncoupled MIT and DIT.
| Parameter | Thyroxine (T4) | Triiodothyronine (T3) | Reverse T3 (rT3) |
|---|---|---|---|
| Iodine Positions | 3, 5 (inner) & 3′, 5′ (outer ring) | 3, 5 (inner) & 3′ (outer ring) | 3 (inner) & 3′, 5′ (outer ring) |
| Daily Production | ~85–100 μg/day (100% thyroidal) | ~30 μg/day (~80% via peripheral DIO1/2) | ~25–30 μg/day (~98% via DIO3) |
| Serum Total & Free % | 5–12 μg/dL; 0.02–0.03% free | 80–180 ng/dL; 0.3–0.4% free | 10–24 ng/dL; ~0.1–0.2% free |
| Plasma Half-Life | ~6.5–7.5 days (tight TBG binding) | ~18–24 hours (rapid turnover) | ~3–4 hours (rapid clearance) |
| Nuclear TR Affinity | Kd ≈ 2–5 nM (prohormone) | Kd ≈ 0.1–0.3 nM (active ligand) | Negligible binding (inactive) |
Neuroendocrine Regulation and Peripheral Deiodinases: How Is Hormone Activity Tuned?
The Hypothalamic–Pituitary–Thyroid (HPT) Axis and Plasma Binding Proteins
Systemic thyroid status is governed by the Hypothalamic–Pituitary–Thyroid (HPT) axis. Parvocellular neurons in the hypothalamic paraventricular nucleus (PVN) secrete thyrotropin-releasing hormone (TRH) into the portal plexus to stimulate anterior pituitary thyrotrophs to secrete thyroid-stimulating hormone (TSH). TSH binds Gαs-coupled TSH receptors (TSHR) on thyrocytes, activating cAMP and Protein Kinase A. Circulating free T4 and T3 enter pituitary thyrotrophs and hypothalamic tanycytes, where T3 binds TRβ2 to repress TSHB and TRH transcription. In plasma, >99.7% of hormone binds thyroxine-binding globulin (TBG, ~70–75%), transthyretin (TTR, ~15–20%), and albumin (~5–10%), leaving 0.03% of T4 and 0.3% of T3 free.
Selenocysteine Iodothyronine Deiodinases (DIO1, DIO2, and DIO3)
Target tissues regulate intracellular T3 via three iodothyronine deiodinases (DIO1, DIO2, DIO3) containing the 21st amino acid selenocysteine (Sec). DIO2 (in brain astrocytes, pituitary, brown fat, and muscle) catalyzes outer-ring (5′) deiodination of T4 into active T3 for nuclear receptor saturation. Conversely, DIO3 (in placenta, fetus, and neurons) catalyzes inner-ring (5) deiodination, inactivating T4 to reverse T3 (rT3) and T3 to 3,3′-T2. DIO1 (in liver and kidney) produces plasma T3 and clears rT3; during fasting or sepsis (non-thyroidal illness), cytokines suppress DIO1 and induce DIO3, lowering serum T3 and raising rT3.
Nuclear Receptor Signaling and Physiological Actions: How Does T3 Regulate Genes?
TRα/TRβ–RXR Heterodimers and Chromatin Remodeling
After entering target cells via transporters such as MCT8 (SLC16A2; mutated in Allan–Herndon–Dudley syndrome) and OATP1C1, T3 binds nuclear Thyroid Hormone Receptors encoded by THRA (TRα1, predominant in heart, skeletal muscle, bone, and brain) and THRB (TRβ1 in liver and kidney; TRβ2 in pituitary, hypothalamus, retina, and cochlea). TRs reside constitutively inside the nucleus bound as heterodimers with the Retinoid X Receptor (RXR) on Thyroid Hormone Response Elements (TREs, 5′-AGGTCA-3′ DR4 repeats). In the unliganded state, the TR–RXR aporeceptor binds corepressors (NCoR1 and SMRT), recruiting histone deacetylase 3 (HDAC3) to condense chromatin and actively repress basal transcription. Binding of T3 (Kd ≈ 0.1 nM) repositions Helix 12, releasing NCoR/SMRT–HDAC3 and recruiting coactivators (SRC-1, CBP/p300) with histone acetyltransferase (HAT) activity to activate target gene transcription.
Basal Metabolic Rate, Cardiovascular Output, and Amphibian Metamorphosis
Through transcriptional activation of nuclear and mitochondrial genes, T3 regulates four systemic programs: (1) Thermogenesis and Basal Metabolic Rate, upregulating Na+/K+-ATPase, sarcoplasmic reticulum SERCA Ca2+-ATPases, mitochondrial respiratory complexes, and brown-fat Uncoupling Protein 1 (UCP1) to accelerate cellular respiration and heat production; (2) Cardiovascular Hemodynamics (via cardiac TRα1), upregulating fast α-myosin heavy chain (MYH6), SERCA2a, and β1-adrenergic receptors to increase heart rate (positive chronotropy) and contractility (positive inotropy); (3) Hepatic Lipid Metabolism (via TRβ1), inducing LDL receptors (LDLR) and cholesterol 7α-hydroxylase (CYP7A1)—explaining why hypothyroidism elevates plasma LDL cholesterol; and (4) CNS Myelination and Amphibian Metamorphosis, driving fetal neuronal migration and oligodendrocyte myelination in mammals while triggering tail resorption and limb outgrowth in amphibian tadpoles.
Frequently Asked Questions
Why is serum TSH the most sensitive blood test for diagnosing thyroid disease?
In an intact hypothalamic–pituitary–thyroid axis, anterior pituitary thyrotrophs respond to small fluctuations in circulating free T4 via an inverse log-linear relationship: a 2-fold decrease or increase in serum free T4 triggers an inverse 10- to 100-fold change in pituitary TSH secretion. Consequently, in primary hypothyroidism (such as autoimmune Hashimoto's thyroiditis with anti-TPO antibodies, or dietary iodine deficiency), serum TSH rises above the normal reference range (0.4–4.5 mIU/L) into the 10–100 mIU/L range even before free T4 falls below normal. Conversely, in primary hyperthyroidism (Graves' disease), negative feedback suppresses serum TSH to <0.01 mIU/L.
How can both iodine deficiency and sudden iodine excess cause a goiter (the Wolff–Chaikoff effect)?
In chronic iodine deficiency (<50 μg/day intake), low T4 synthesis diminishes negative feedback on the pituitary, causing elevated TSH, which stimulates follicular hypertrophy and hyperplasia into an endemic goiter. Paradoxically, when an individual receives an acute high dose of iodide (such as potassium iodide, amiodarone, or iodinated CT contrast), once intrathyroidal iodide exceeds ~10−3 M, it forms iodolipids that transiently inhibit DUOX2 H2O2 generation and TPO organification for 24 to 48 hours—the Wolff–Chaikoff effect (1948). Healthy glands escape within 2 days by downregulating NIS, whereas patients with autoimmune thyroiditis may develop iodide-induced hypothyroidism.
Why do patients with hypothyroidism receive levothyroxine (T4) rather than active T3 (liothyronine)?
Although T3 is the active hormone that binds nuclear TRα and TRβ receptors with 10- to 15-fold higher affinity than T4, oral liothyronine (T3) has a short plasma half-life (~18 to 24 hours) and lower TBG binding, causing supraphysiological serum T3 peaks (risking tachycardia and atrial arrhythmias) followed by troughs. In contrast, oral levothyroxine (synthetic L-T4) binds tightly to TBG, giving it a smooth ~7-day biological half-life that maintains steady-state plasma reservoirs from a single daily dose, allowing each target tissue to regulate its own intracellular T3 concentration via local DIO2 and DIO3 deiodinase activity.
References
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- Bianco, A. C., Salvatore, D., Gereben, B., Berry, M. J., & Larsen, P. R. (2002). "Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases." Endocrine Reviews, 23(1), 38–89. https://doi.org/10.1210/edrv.23.1.0455
- Mullur, R., Liu, Y.-Y., & Brent, G. A. (2014). "Thyroid hormone regulation of metabolism." Physiological Reviews, 94(2), 355–382. https://doi.org/10.1152/physrev.00030.2013
- Dai, G., Levy, O., & Carrasco, N. (1996). "Cloning and characterization of the thyroid iodide transporter." Nature, 379(6564), 458–460. https://doi.org/10.1038/379458a0