A healthy woman at 20 weeks of gestation has a total serum T4 of 16 µg/dL (nonpregnant reference 5 to 12), normal free T4, and TSH of 1.2 µU/mL. Which change best explains her elevated total T4?

• A Reduced deiodination of T4 by placental type 3 deiodinase
• B Placental secretion of human chorionic gonadotropin stimulating the thyroid
• C Estrogen-driven increase in hepatic thyroxine-binding globulin synthesis ✓
• D Increased volume of distribution lowering free hormone fraction

Explanation

Estrogen increases hepatic sialylation and synthesis of thyroxine-binding globulin, expanding the bound reservoir and raising total T4 while free T4 and TSH remain normal because feedback depends on the free fraction. hCG does stimulate the thyroid in early pregnancy, but that raises free T4 and suppresses TSH transiently, which is not the pattern here, killing option B.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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