IODOTHYRONINE DEIODINASE ACTIVITIES IN FETAL-RAT TISSUES AT SEVERAL LEVELS OF IODINE DEFICIENCY - A ROLE FOR THE SKIN IN 3,5,3'-TRIIODOTHYRONINE ECONOMY

Iodothyronine deiodinases, types I, II, and III (D1, D2, and D3) activ ities were measured in tissues of fetal rats, at 18 and 21 days of ges tation, at several levels of iodine deficiency (ID): mild ID diet (MID ) and moderately severe ID, MID + 0.005% perchlorate (MID+P). D2 was p resent in fetal skin, increased between days 18 and 21, and also in MI D and MID+P. In skin, D3 increased during ID at day 18, whereas there was a decrease at day 21. Skin T-4 decreased in MID and MID+P, showing an inverse relationship with D2. Skin T-3 decreased at day 18 in MID and MID+P but increased at day 21, probably because of the increased D 2 and decreased D3, maintaining T-3 concentrations. No effect of ID wa s observed on hepatic D1. D2 increased in brain and brown adipose tiss ue at day 21 in MID-P. No changes were found in maternal placental D2 and D3, but D2 and D3 increased in the fetal placenta at day 18 in MID +P. A higher level of D2 is present in fetal skin than in the brain. A s the activity is increased, in even mild ID land already at 18 days) it can be concluded that skin D2 is likely to be of considerable physi ological importance, at least for fetal thyroid hormone economy, by co ntributing to the intracellular T-3 content of the skin and, possibly, to the plasma T-3.