Only about 20 percent of circulating triiodothyronine is secreted directly by the thyroid; the remainder arises peripherally. Which deiodinase isoform, expressed predominantly in liver and kidney, generates most of the circulating T3 pool?

• A Type 2 iodothyronine deiodinase
• B Type 1 iodothyronine deiodinase (outer ring) ✓
• C Type 3 iodothyronine deiodinase
• D Type 4 iodothyronine deiodinase

Explanation

Type 1 deiodinase in liver and kidney performs outer-ring deiodination of T4 to supply most of the circulating T3. Type 2 deiodinase generates T3 locally inside brain, brown fat, and pituitary cells rather than contributing to plasma, and type 3 catalyzes inner-ring deiodination to inactive reverse T3. Only three mammalian deiodinase isoforms exist, so option D names an entity that does not exist.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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