Conversion of thyroxine to triiodothyronine and reverse triiodothyronine in human placenta and fetal membranes.

Conversion of thyroxine (T4) to 3,5,3'-triiodothyronine (T3) and reverse 3,3',5'-triiodothyronine (rT3) was measured in vitro in human placenta and fetal membranes. T4 (5 micrograms/ml) was incubated in 0.15 mol/l phosphate buffer with tissue homogenates for 2 h at 37 degrees C, and the T3 and rT3 generated were determined in ethanol extract using RIA methods. The placenta and chorion homogenates converted more T4 to T3 than to rT3; the placenta was more active than the chorion. In both tissues the highest converting activity was found in microsomal fractions.