Antithyroid treatment of maternal hyperthyroidism during lactation.

Eleven pregnant women were treated for hyperthyroidism with carbimazole (CZ) and one with propylthiouracil (PTU). Based upon a previous study it was decided that lactation should be permitted if the dose required after delivery did not exceed 15 mg of CZ or 150 mg of PTU. In the patients studied here the daily dose of CZ varied from 5 to 15 mg and that of PTU was 125 mg. TSH was measured in cord blood and in the blood of the newborn infants usually after 2 and 3 weeks of lactation. Serum T4 was measured serially in the infants' blood from day 4 up to 21 day of age, at least. In all instances the TSH concentration in cord blood remained below 45 mU/l the level used in screening for neonatal hypothyroidism. Serum TSH and T4 were all within the appropriate reference limits during the 3 weeks of study with only one exception. In the infant whose mother was treated with PTU the serum T4 measured 5 d after birth was slightly below the lower limit but later returned to normal. Since serum TSH and T4 did not deviate from the reference range in newborn infants during lactation, we conclude that breast-feeding can be permitted if the daily dose of CZ does not exceed 15 mg (or 150 mg of PTU) and if facilities are available for measuring neonatal serum TSH and T4.