Type 1 and type 2 iodothyronine deiodinases contain selenocysteine at their active sites. Selenium deficiency therefore directly impairs which conversion?

• A Conversion of T4 to triiodothyronine (T3) ✓
• B Conversion of T4 to reverse T3
• C Iodination of thyroglobulin tyrosine residues
• D Coupling of diiodotyrosine residues to form T4

Explanation

Deiodinases remove iodine from thyroxine, and their active-site selenocysteine explains why selenium status influences thyroid economy. Selenium deficiency lowers T3 production from T4, producing a compensatory rise in T4 and TSH. Organification of iodine and the coupling reaction occur inside thyroglobulin and depend on thyroid peroxidase, a haem-containing enzyme, not on selenium, which kills options C and D.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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