For many years, breast-feeding was forbidden if antithyroid drugs were being used. Recently, limited studies have shown the relative safety of propylthiouracil and methimazole (MMI). It is not known whether MMI therapy of lactating mothers for 1 yr is safe for breast-fed infants and does not cause alterations in thyroid function and intellectual development. Between 1988 and 1998, 139 thyrotoxic lactating mothers and their infants were studied. Fifty-one thyrotoxic lactating mothers were treated with MMI during pregnancy, and MMI was continued during breast-feeding. Eighty-eight mothers were given 10 mg MMI (n 46) or 20 mg MMI (n = 42) daily for 1 month, 10 mg daily for the second month, and 5-10 mg daily thereafter. Serum T4, T3, and TSH concentrations were measured in thyrotoxic lactating mothers and their infants, before and at 1, 2, 4, 8, and 12 months. Serum MMI was measured in the infants of thyrotoxic lactating mothers taking 20 mg MMI. Thyroid function, urinary iodine, thyroid antibodies, intelligence quotient (IQ), verbal and functional components (Wechsler and Goodenough tests) were performed on 14 children of thyrotoxic lactating mothers between 48 and 74 months of age and on 17 controls. Mean +/- SD of FT4I in thyrotoxic lactating mothers treated with 10 mg MMI for 1 month decreased from 19.4 +/- 4.1 to 11.6 +/- 4.4 and from 20.5 +/- 4.7 to 9.8 +/- 1.5 when treated with 20 mg MMI. Values for FT3I decreased from 462 +/- 52 to 194 +/- 52 with 10 mg MMI and from 481 +/- 92 to 171 +/- 38 with 20 mg MMI. FT4I and FT3I were normal from the third to the twelfth months. In all infants FT4I, FT3I, and TSH concentrations were normal before and up to 12 months of MMI therapy in their lactating mothers. The lowest T4 and T3 values were 108 and 1.87 nmol/L, and the highest TSH value was 4.0 mU/L. Serum MMI levels in infants were less than 0.03 microg/mL. Six mothers receiving 20 mg MMI had increased serum TSH concentrations ranging from 26-135 mU/L after 1 month of treatment. Their infants were euthyroid with serum TSH values less than 2.6 mU/L. At 48-74 months of age, height, weight, FT4I, FT3I, TSH, and antithyroid antibody titers were not different than controls. The mean IQ was 107 +/- 14 vs. 106 +/- 16 (Goodenough test) and 103 +/- 10 vs. 103 +/- 16 (Wechsler test) for infants of thyrotoxic lactating mothers and control infants, respectively. Similarly, there was no difference in verbal and performance IQ and their components between infants of thyrotoxic lactating mothers and control children. No deleterious effects occur in thyroid function and physical and intellectual development of breast-fed infants whose lactating mothers were treated with doses of MMI up to 20 mg daily.