Ontogeny of the generation of diiodothyronines (3,3'-T2 and 3',5'-T2) from triiodothyronines in the liver and kidney during the fetal life of the pig.

The present study was conducted to obtain information on the generation of diiodothyronines from triiodothyronines (3,3'-T2 from T3, and rT3, and 3',5'-T2 from rT3) as a result of the activity of the tissue monodeiodinase enzymes (MD) in the liver and kidney of pig fetuses and their mothers between 32 and 113 days of gestation. T3-5-MD activity in the fetal kidney during the gestational period was stable and higher than in the liver and in the maternal kidney. In contrast, T3-5-MD activity of the liver was 3-4 times lower in fetal than in maternal tissue in the first half of pregnancy, and in the second half of pregnancy 3,3'-T2 production from T3 in the maternal liver was equal to, or lower than, that in the fetal liver. The activity of MD deiodinating rT3 (3,3'-T2 and 3',5'-T2 generation) increased significantly in fetal liver and kidney in the last 2-3 weeks of pregnancy and was higher than in maternal tissues. In both tissues examined the inner ring deiodinating activity (IRD) was 5-10 times lower as compared to the outer ring (ORD).