A healthy pregnant woman at 20 weeks gestation has a total serum T4 of 16 µg/dL (nonpregnant reference 5 to 12). Which physiologic change best explains this finding?

• A Placental secretion of hCG suppressing TSH below normal
• B Estrogen-stimulated hepatic synthesis of thyroxine-binding globulin ✓
• C Increased conversion of T4 to reverse T3 by type 3 deiodinase
• D Reduced renal clearance of iodide raising intrathyroidal hormone stores

Explanation

Rising estrogen increases hepatic thyroxine-binding globulin synthesis during pregnancy. The enlarged binding protein pool raises total T4 even though the free fraction and TSH remain normal and the woman stays euthyroid. This is the classic situation where only free T4 is interpretable. Option A describes transient early-pregnancy hCG stimulation of the thyroid, which lowers TSH slightly but does not explain an isolated rise in total T4.

Reference: Williams Textbook of Endocrinology, 14th ed.

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