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.
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