3,3′,5′-Triiodothyronine (Reverse T3) and 3,3′,5-Triiodothyronine)(T3) in Fetal and Adult Sheep: Studies of Metabolic Clearance) Rates, Production Rates, Serum Binding, and Thyroidal Content Relative to Thyroxine

摘要: To examine the mechanism(s) responsible for high serum concentration of 3,3',5'-triiodothyronine (reverse T3, rT3) and low 3,3',5-triiodothyronine (T3) in fetus, we studied metabolic clearance rates (MCR) production (PR) rT3, thyroxine (T4) adult nonpregnant sheep fetuses utero. The mean fetal MCR-rT3 was significantly lower than that sheep, PR-rT3 higher. MCR-T3 higher than, PR-T3 similar to sheep. MCR-T4 PR-T4 were both corresponding values ratios (rT3/T4) However, ratio (T3/T4) much not attributable binding rT3. On basis thyroidal content kinetics iodothyronines, it estimated whereas secretion may account nearly all T3 (or PR-T3) fetus about 50% adults, accounts only 3% rT3 PR-rT3) results suggest a) elevated is due its decreased increased by mono-deiodination T4, b) T4. In addition, on discordant changes from appears there exist two separate, apparently specific, iodothyronine deiodinating activities--one cleaving iodine atom at 5'-position other acting 5-position T4 molecule; 5'-iodothyronine activity reduced fetus.