Decreased outer ring monodeiodination of thyroxine and reverse triiodothyronine in the fetal and neonatal rat.

Abstract

Studies of the deiodination of T4 and rT3 were carried out in the perinatal rat. As compared to the adult male, the in vitro hepatic homogenate conversion of T4 to T3 was strikingly reduced in the 14 and 19 day-old fetal rat, increased progressively after birth, and reached adult male values by day 5. T4 or T3 administration enhanced and starvation decreased in vitro hepatic T3 generation from T4 in the neonatal rat. A highly significant decrease in in vitro degradation of [l25I]rT3 was also observed in the perinatal rat and adult values were reached by day 5. In vivo disposal of iv administered [125I]rT3 was studied in perinatal rats. Ten minutes after iv [125I]r3, the percentage of total 125I- remaining in the serum as [125I]r3 in the 20-day-old fetal rat was 70% and decreased progressivel towards adult values throughout the neonatal period. The metabolic clearance rate of rT3 was much slower in the 10-day-old neonate as compared to the adult rat (1.4 ml/min-100 g vs. 6.7). In comparison to adult values...