What proportion of circulating triiodothyronine (T3) in a healthy adult is derived from peripheral conversion of T4 rather than direct thyroidal secretion?
- A About 20 percent
- B About 80 percent ✓
- C About 50 percent
- D About 95 percent
Correct answer: B. About 80 percent
Explanation
The thyroid secretes mostly T4, which is biologically a prohormone. Roughly 80 percent of plasma T3 is generated extrathyroidally by 5-prime-deiodinase enzymes removing an iodine from the outer ring of T4, mainly in liver and kidney. The remaining fraction is secreted directly by the follicular cells. This is why drugs like propylthiouracil have an added benefit of blocking peripheral conversion, unlike methimazole.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.