Placental Transfer of L-Thyroxine and Tri-iodo-L-thyronine in Rats Daring Late Pregnancy. ∗

Abstract

Summary

The placental transfer of L-thyroxine and triiodo-L-thyronine during late pregnancy in the rat was studied using radioactive hormones. It was found that both hormones cross the placental barrier into the fetus. The concentration of T₃ in fetal serum on day 20 of pregnancy was twice that of maternal serum, but was equivalent to maternal serum on day 21. The concentration of T₄ in fetal serum was one-half of that in maternal serum on day 20 and 21 of pregnancy. Both T₃ and T₄ were concentrated by the placenta. T₃ was concentrated 15-fold more by the placenta than was T₄ from an equivalent amount of hormone in maternal serum.