A 35-year-old woman with Graves' hyperthyroidism is 8 weeks pregnant. Which antithyroid drug is preferred in the FIRST trimester to minimise fetal risk?

• A Methimazole, as it has the best safety record in all trimesters
• B Carbimazole, which does not cross the placenta
• C Radioactive iodine at the lowest available dose
• D Propylthiouracil (PTU), due to lower risk of embryopathy compared to carbimazole/methimazole ✓

Explanation

PTU is preferred in the first trimester because methimazole/carbimazole is associated with aplasia cutis, choanal atresia, and the methimazole embryopathy syndrome. PTU carries a lower teratogenic risk during organogenesis. From the second trimester onward, methimazole is generally preferred due to PTU's risk of maternal hepatotoxicity. 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.