Previous studies have characterized the pharmacology of propylthiouracil (PTU) in normal and hyperthyroid subjects, but there is little information available regarding PTU pharmacokinetics in pregnant hyperthyroid women. We investigated the serum PTU response to an oral dose of PTU in six hyperthyroid pregnant women both ante- and postpartum. The serum PTU profile during the third trimester of pregnancy was qualitatively similar to that in nonpregnant subjects, but serum PTU concentrations were consistently lower in the late third trimester compared with postpartum values. Cord serum PTU concentrations were consistently higher than simultaneously obtained maternal serum PTU concentrations, suggesting slower PTU clearance in the fetus. There was a significant inverse correlation (r = -0.92; P = 0.026) between the maternal serum PTU area under the curve in the third trimester and the cord serum free T4 index.