Chopra I J, Sack J, Fisher D A
Endocrinology. 1975 Nov;97(5):1080-8. doi: 10.1210/endo-97-5-1080.
To examine the mechanism(s) responsible for high serum concentration of 3,3',5'-triiodothyronine (reverse T3, rT3) and low serum concentration of 3,3',5-triiodothyronine (T3) in the fetus, we studied metabolic clearance rates (MCR) and production rates (PR) of rT3, T3, and thyroxine (T4) in adult nonpregnant sheep and sheep fetuses in utero. The mean fetal MCR-rT3 was significantly lower than that in adult sheep, and the mean fetal PR-rT3 significantly higher. The mean fetal MCR-T3 was higher than, and the mean fetal PR-T3 similar to that in adult sheep. The mean fetal MCR-T4 and PR-T4 were both significantly higher than the corresponding values in adult sheep. The ratios of mean PR-rT3 to PR-T4 (rT3/T4) were similar in fetal and adult sheep. However, the ratio of mean PR-T3 to PR-T4 (T3/T4) in the fetal sheep was much lower than that in the adult sheep. The low fetal MCR-rT3 was not attributable to high serum binding of rT3. On the basis of the thyroidal content and kinetics of iodothyronines, it was estimated that whereas thyroidal secretion may account for nearly all of serum T3 (or PR-T3) in the fetus and about 50% of serum T3 in adults, it accounts for only about 3% of the serum rT3 (or PR-rT3) in both fetal and adult sheep. The results suggest a) that elevated serum rT3 in the fetus is due to its decreased clearance and increased production by mono-deiodination of T4, and b) that low serum T3 in the fetus is due to its increased clearance and decreased production by mono-deiodination of T4. In addition, on the basis of discordant changes in the production of T3 and rT3 from T4, it appears that there may exist two separate, apparently specific, iodothyronine deiodinating activities--one cleaving the iodine atom at the 5'-position and the other acting in the iodine atom at the 5-position of the T4 molecule; 5'-iodothyronine deiodinating activity is apparently reduced in the fetus.
为研究胎儿血清中3,3',5'-三碘甲腺原氨酸（反式T3，rT3）浓度高而3,3',5-三碘甲腺原氨酸（T3）浓度低的机制，我们研究了成年未孕绵羊及子宫内绵羊胎儿体内rT3、T3和甲状腺素（T4）的代谢清除率（MCR）和生成率（PR）。胎儿的平均MCR-rT3显著低于成年绵羊，而胎儿的平均PR-rT3显著高于成年绵羊。胎儿的平均MCR-T3高于成年绵羊，且胎儿的平均PR-T3与成年绵羊相似。胎儿的平均MCR-T4和PR-T4均显著高于成年绵羊的相应值。胎儿和成年绵羊的平均PR-rT3与PR-T4之比（rT3/T4）相似。然而，胎儿绵羊的平均PR-T3与PR-T4之比（T3/T4）远低于成年绵羊。胎儿较低的MCR-rT3并非归因于rT3的高血清结合。根据甲状腺内碘甲腺原氨酸的含量和动力学，估计甲状腺分泌在胎儿体内可能几乎占血清T3（或PR-T3）的全部，在成年人体内约占血清T3的50%，而在胎儿和成年绵羊体内仅占血清rT3（或PR-rT3）的约3%。结果表明：a）胎儿血清rT3升高是由于其清除率降低以及通过T4的单脱碘作用生成增加；b）胎儿血清T3降低是由于其清除率增加以及通过T4的单脱碘作用生成减少。此外，基于T4生成T3和rT3的变化不一致，似乎可能存在两种独立的、明显具有特异性的碘甲腺原氨酸脱碘活性——一种在T4分子的5'-位裂解碘原子，另一种作用于T4分子的5-位碘原子；胎儿体内5'-碘甲腺原氨酸脱碘活性明显降低。
