The Placental Transfer of Propylthiouracil, Methimazole and Carbimazole

Abstract

The placental transfer of 35S-labeled methimazole (MMI), carbamazole and propylthiouracil (PTU) [antithyroid drugs for the treatment of thyrotoxicosis associated with pregnancy] was examined in the rat in late pregnancy and in patients undergoing therapeutic abortion. Although rapid equilibrium of fetal and maternal serum radioactivity (FS:MS ratio 1:1) occurred after i.v. administration of 35S-carbimazole or 35S-MMI in rats, a persistent fetal to maternal ratio of less than 1 was observed after 35S-PTU administration. Results from human studies after a single oral dose indicate that, as in the rat, the placenta appeared to be more permeable to 35S-MMI than to 35S-PTU as shown by the marked difference in fetal serum:maternal serum ratios and amounts accumulated in the fetus. Localization of radioactivity in the human fetal thyroid was also observed after administration of 35S-labeled MMI, carbimazole or PTU.