The placental transfer of propylthiouracil, methimazole and carbimazole

The placental transfer of 35S-labelled methimazole (MMI), carbimazole and propylthiouracil (PTU) has been 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 iv administration of 35S-carbimazole or 35S-MMI in rats, a persistent fetal to maternal ratio of less than one 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-labelled MMI, carbimazole or PTU.