A 26-year-old woman who is 32 weeks’ pregnant with her first child presents to her obstetrician for a routine visit. Her pregnancy has been uneventful and she has been feeling well, although somewhat anxious over the arrival of her first child. She has blood drawn for glucose levels following a 1-hour glucose tolerance test, but the laboratory mistakenly runs the sample for thyroid hormones; testing reveals the following values:

Total thyroxine: 20.5 µg/dL
Free thyroxine: 7.0 ng/dL
Total triiodothyronine: 280 ng/dL
Thyroid-stimulating hormone: 2.2 µU/mL

Given her thyroid hormone levels, what is the most appropriate next step in management?

A. Nothing; this woman does not have hyperthyroidism
B. Nothing; asymptomatic hyperthyroidism does not require treatment during pregnancy
C. Treatment with propylthiouracil because hyperthyroidism can have deleterious effects on the developing fetus
D. Treatment with surgery because hyperthyroidism can have deleterious effects on the developing fetus
E. Treatment with thyroxine because hyperthyroidism can have deleterious effects on the developing fetus

The correct answer is A. The laboratory values shown here indicate an increased total thyroxine (T4) and triidothyronine (T3) and normal free T4 and thyroid-stimulating hormone levels. During pregnancy, thyroxine-binding globulin increases secondary to increased hepatic synthesis, causing falsely elevated total T3 and T4 levels. This is not a true state of hyperthyroidism because levels of active, or free, T4 are unchanged.

Answer B is not correct. Uncontrolled maternal hyperthyroidism has been associated with stillbirth, spontaneous abortion, premature labor, birth defects, low birth weight, and preeclampsia. Excess thyroid hormone has been shown to slow fetal growth, so mothers with hyperthyroidism must be carefully treated and closely monitored.

Answer C is not correct. If this woman had true hyperthyroidism as indicated by increased free T4 and decreased thyroid-stimulating hormone, propylthiouracil treatment would be indicated.

Answer D is not correct. Surgery would be used as a last resort for hyperthyroidism following failed medical management.

Answer E is not correct. Thyroxine is the treatment of choice for hypothyroidism, not hyperthyroidism. Giving patients with hyperthyroidism even more thyroid hormone can precipitate thyrotoxicosis.

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