Minimal placental transfer of L-thyroxine (T4) in the rat

Studies of L-thyroxine T4 kinetics and placental transfer were conducted in pregnant rats during the last days of gestation using radioiodine labeled and unlabeled T4. The maternal T4 secretion rate was 53.0 +/- 6.9 ng/hr/100 g body wt. The placental transfer rate was 0.463 +/- 0.196 x 10(-3)/hr at 18 days and 1.516 +/- 0.147 x 10(-3)/hr at 20 days. From these results, we calculated that at 20 days of gestation, 80 +/- 8 pg/hr were transfered to the fetus, less than 1% of the fetal production rate. Inasmuch as we have demonstrated that little placental transfer of T4 occurred during these studies, we conclude that the fetal rat hypothalamo-pituitary-thyroid axis develops autonomously.