A 40-year-old with Graves' disease is 10 weeks pregnant. FT4 is 2.8 times ULN. Which antithyroid drug and dose strategy best balances fetal and maternal risk in the first trimester?
- A Propylthiouracil (PTU) at the lowest effective dose, targeting maternal FT4 in the upper third of normal ✓
- B Carbimazole 40 mg/day block-and-replace with levothyroxine
- C Methimazole 30 mg/day and monitor fetal growth by ultrasound every 4 weeks
- D Radioiodine therapy with appropriate shielding of fetal thyroid
Correct answer: A. Propylthiouracil (PTU) at the lowest effective dose, targeting maternal FT4 in the upper third of normal
Explanation
In the first trimester, PTU is preferred over methimazole/carbimazole because methimazole carries a risk of embryopathy (aplasia cutis, choanal atresia). PTU is used at the lowest dose needed to keep maternal FT4 in the upper-normal range to prevent fetal hypothyroidism, since antithyroid drugs cross the placenta. Block-and-replace regimens are contraindicated in pregnancy because levothyroxine does not cross the placenta adequately to protect the fetus. Radioiodine is absolutely contraindicated in pregnancy.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.