Which thionamide is preferred for the first trimester in a pregnant patient with hyperthyroidism if therapy is needed?

• A. Propylthiouracil
• B. Methimazole
• C. Carbimazole
• D. Levothyroxine

Answer: (A)

In the first trimester, treating hyperthyroidism with a thionamide, propylthiouracil is preferred because it carries a lower risk of fetal congenital malformations that have been linked to methimazole and carbimazole, such as esophageal or choanal atresia and other scalp or facial defects. Propylthiouracil effectively reduces thyroid hormone production, and in the first 12 weeks its use minimizes the chance of birth defects while still controlling maternal hyperthyroidism. After the first trimester, clinicians often switch to methimazole or carbimazole to reduce the risk of maternal hepatotoxicity associated with propylthiouracil during longer-term therapy, while continuing to monitor thyroid levels to keep them in a safe range for both mother and baby. Levothyroxine, by contrast, treats hypothyroidism and is not used to treat hyperthyroidism.