Which antithyroid drug is often preferred in the first trimester of pregnancy due to teratogenic risks?

• A. Methimazole
• B. Propylthiouracil
• C. Carbimazole
• D. Radioactive iodine

Answer: (B)

When managing hyperthyroidism in early pregnancy, the goal is to minimize fetal thyroid disruption and birth defects. Methimazole and its analog carbimazole carry a known risk of congenital malformations if used in the first trimester, including scalp defects and other anomalies. Propylthiouracil, while it can cause rare liver toxicity in the mother, has a much lower risk of these teratogenic effects, so it is preferred during the first trimester. After the first trimester, many clinicians switch to methimazole/carbimazole to reduce the risk of PTU-related hepatotoxicity in the mother. Radioactive iodine is avoided in pregnancy because it destroys fetal thyroid tissue. Therefore, the drug chosen for first-trimester use due to teratogenic risk is propylthiouracil.