Thyroid Hormone Production: Essential Components and Process

Thyroid hormone synthesis (thyroxine/T4 and triiodothyronine/T3) is a complex, multi-step process requiring precise coordination of substrates, enzymes, transporters, and regulatory signals.

Essential Components

1. Iodide (I⁻)

• Dietary intake (iodized salt, seafood, dairy). RDA: 150 µg/day adults; 220–290 µg pregnancy/lactation.
• Absorbed in GI tract → plasma iodide pool.

2. Sodium-Iodide Symporter (NIS)

• Basolateral membrane protein on thyrocytes.
• Active transport: 2 Na⁺ + I⁻ into cell (driven by Na⁺/K⁺-ATPase gradient).
• Concentrates iodide 20–50× plasma levels.
• Also in salivary glands, gastric mucosa, lactating breast, placenta.
• Inhibited by thiocyanate, perchlorate, pertechnetate.

3. Thyroglobulin (Tg)

• Large (660 kDa) dimeric glycoprotein synthesized in thyrocytes.
• Secreted into follicular lumen (colloid) – storage form.
• Tyrosine residues on Tg serve as iodination sites.

4. Thyroid Peroxidase (TPO)

• Apical membrane heme enzyme.
• Catalyzes two reactions using H₂O₂ (from DUOX2):
• Iodination: I⁻ + tyrosine on Tg → MIT (monoiodotyrosine) + DIT (diiodotyrosine).
• Coupling: DIT + DIT → T4; MIT + DIT → T3 (within Tg molecule).

5. Thyrotropin (TSH)

• Pituitary glycoprotein hormone (α + β subunits).
• Binds TSHR (GPCR) on basolateral membrane → cAMP/PKA & PLC/IP3/DAG pathways.
• Stimulates: NIS expression, Tg synthesis, TPO activity, H₂O₂ generation, endocytosis of colloid, hormone secretion, thyrocyte growth.
• Regulated by TRH (hypothalamus) and negative feedback by T4/T3.

6. Deiodinases (D1, D2, D3)

• Selenoproteins that activate/inactivate thyroid hormones peripherally.
• D1 (liver, kidney, thyroid): T4 → T3 (activation); rT3 → T2 (clearance).
• D2 (brain, pituitary, brown fat, muscle): T4 → T3 (local activation); critical for cerebral T3.
• D3 (placenta, CNS, skin): T4 → rT3 (inactivation); T3 → T2; protects tissues from excess T3.

7. Energy and Nutritional Status

• ATP for NIS, Tg synthesis, H₂O₂ generation.
• Adequate selenium (for deiodinases, GPx), iron (for TPO heme), zinc, vitamin A.
• Goitrogens (thiocyanates in cassava, crucifers; flavonoids) can impair steps if iodine marginal.

Summary Pathway

I⁻ (diet) → NIS → Thyrocyte → Apical TPO/H₂O₂ → Tg iodination/coupling → Colloid storage → TSH-stimulated endocytosis → Lysosomal proteolysis → T4/T3 secretion → Bloodstream → Tissue deiodination → Nuclear receptor action.

Disruption at any step causes hypothyroidism (or goiter if TSH-driven hyperplasia).