A healthy woman at 20 weeks of gestation has a total serum T4 of 16 microgram/dL (elevated for nonpregnant reference range), free T4 of 1.2 ng/dL (normal), and TSH of 1.8 mIU/L (normal). What is the physiological basis of her raised total T4?

• A Increased peripheral conversion of T4 to T3 raising total hormone load
• B Stimulation of the thyroid gland by placental hCG causing true hyperthyroidism
• C Reduced renal clearance of T4 because of expanded plasma volume
• D Estrogen-stimulated hepatic synthesis of thyroxine-binding globulin ✓

Explanation

Estrogen increases hepatic synthesis of thyroxine-binding globulin, so more total T4 circulates bound to protein while the free hormone fraction, which determines feedback and tissue action, stays normal. The normal free T4 and TSH confirm euthyroid status. Although hCG does rise in pregnancy and can transiently stimulate the thyroid in early gestation, that produces a fall in TSH, and it does not explain isolated elevation of total T4 with normal free T4.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.