Which enzyme mediates inner-ring deiodination of thyroxine, producing the biologically inactive metabolite reverse T3?
- A Type 1 deiodinase
- B Type 2 deiodinase
- C Type 3 deiodinase ✓
- D Iodide peroxidase
Correct answer: C. Type 3 deiodinase
Explanation
Type 3 deiodinase removes iodine from the inner (tyrosyl) ring of iodothyronines, converting T4 to reverse T3 and T3 to diiodothyronine, both inactive. It is abundant in the placenta, fetal tissues and brain, protecting the fetus from excess maternal thyroid hormone. Type 2 and type 1 deiodinases perform outer-ring deiodination and activate T4 to T3, while iodide peroxidase catalyzes organification within the follicular cell.
Reference: Williams Textbook of Endocrinology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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