A 26-year-old woman in her second trimester has a total serum T4 of 15 µg/dL (elevated), free T4 of 1.2 ng/dL (normal), and TSH of 1.4 µU/mL (normal). She feels well and has no tremor or tachycardia. What best explains her elevated total T4?

• A Placental secretion of hCG stimulating maternal thyrotrophs
• B Estrogen-induced increase in hepatic thyroxine-binding globulin synthesis ✓
• C Reduced peripheral conversion of T4 to T3
• D Increased clearance of T4 by the fetal placenta

Explanation

Rising estrogen in pregnancy increases hepatic synthesis of thyroxine-binding globulin, expanding the bound pool and elevating total T4 while free T4 and TSH remain normal. This euthyroid hyperthyroxinemia requires no treatment. Transient hCG-mediated stimulation occurs in early pregnancy but would suppress TSH and raise free T4, which is not seen here.

Reference: Williams Textbook of Endocrinology, 14th ed.

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