## Thyroid Hormone Coupling Reactions **Key Point:** T₄ (thyroxine) is formed by the coupling of two diiodotyrosine (DIT) residues via thyroid peroxidase. This is the predominant thyroid hormone produced by the thyroid gland. ### Coupling Reactions in Thyroid Hormone Synthesis | Coupling Reaction | Product | Iodine Content | Relative Abundance | |---|---|---|---| | MIT + MIT | Monoiodothyronine (T₁) | 1 iodine | Negligible | | MIT + DIT | Triiodothyronine (T₃) | 3 iodines | ~10% of output | | DIT + DIT | Thyroxine (T₄) | 4 iodines | ~90% of output | ### Why DIT + DIT → T₄? 1. **Iodine availability:** DIT has two iodine atoms, allowing formation of a 4-iodine product 2. **Enzyme specificity:** TPO preferentially catalyzes DIT-DIT coupling 3. **Thermodynamic stability:** T₄ is more stable than T₃ and serves as the circulating storage form of thyroid hormone **High-Yield:** The thyroid secretes approximately **80% T₄ and 20% T₃**. However, T₃ is the more biologically active form. Peripheral deiodinase converts T₄ → T₃ in tissues, allowing fine-tuned hormonal control. **Mnemonic:** **"DIT-DIT makes FOUR"** — Two DIT molecules couple to form T₄ (4 iodines). **Clinical Pearl:** In iodine deficiency, the thyroid preferentially produces more T₃ (MIT + DIT coupling) relative to T₄, because T₃ is more potent and requires fewer iodine atoms. This is a compensatory mechanism. [cite:Guyton & Hall Textbook of Medical Physiology Ch 76]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.