Effect of methimazole treatment of maternal thyrotoxicosis on thyroid function in breast-feeding infants.

In 35 infants of lactating mothers with thyrotoxicosis who were receiving 5 to 20 mg methimazole daily, serum levels of thyroxine, triiodothyronine, thyrotropin were within normal ranges 1 month after the start of breast-feeding. Thyroid function in breast-feeding infants of six lactating mothers receiving methimazole, 20 mg for the first, 10 mg for the second, and 5 mg for an additional 4 months, remained normal. These results suggest the safety of methimazole therapy in lactating mothers.