Hyperthyroidism during pregnancy

QUESTION: I have a 33-year-old patient with hyperthyroidism who is 6 weeks pregnant. Her thyroid function is well controlled with a 5-mg dose of methimazole 3 times daily. She was initially treated with propylthiouracil but was switched to methimazole owing to urticaria. I have heard about birth defects in infants whose mothers used methimazole during pregnancy. How safe is it? ANSWER: In North America, propylthiouracil has been the drug of choice for hyperthyroidism during pregnancy. Methimazole is widely used in Europe, South America, and Asia, and is an alternative for patients who cannot tolerate propylthiouracil. Some case reports raised concern about fetal toxicity from methimazole, which is reportedly characterized by aplasia cutis, esophageal atresia, choanal atresia, facial abnormalities, and mental retardation. However, causality is unclear and the overall risk of congenital abnormalities in infants exposed to methimazole in utero was not higher than in those exposed to nonteratogenic drugs in cohort studies. It is important for a pregnant woman to continue methimazole, if necessary, because uncontrolled hyperthyroidism increases the risk of complications such as preterm labour and low birth weight.