Divergent changes of thyroxine-5'-monodeiodination in rat pituitary and liver during maturation.

T4-5'-monodeiodination in pituitary fragments and liver homogenates, serum thyroid hormones, an index of serum thyroid hormone protein binding, and serum TSH were measured in rats aged 2-72 days. Serum T4, T3, and TSH concentrations were lower in the 2-, 4-, and 9-day-old rats than in adult rats. The serum thyroid hormone-binding index was highest in neonatal rats, fell to a nadir at 21 days, and rose thereafter. The intrapituitary T4-5'-monodeiodination rate was 3.5 times the 72-day value in 2- and 4-day-old rats and fell to the 72-day value by 30 days of age. T4-5'-monodeiodination rates in liver homogenates were lower than the adult values in 2-, 4-, and 9-day-old rats, reaching the adult value in 21-day-old rats. When 5 mM dithiothreitol were included in the liver homogenate incubations, the absolute T4-5'-monodeiodination rates were increased, and the differences among 2-, 4-, 9-, 21-, and 72-day-old rats were abolished. Increased rates of intrapituitary T4-5'-monodeiodination may, in part, account for lower serum TSH concentrations in neonatal rats than in adults despite the lower serum T4 concentrations in the former. The differences between changes in pituitary and liver T4-5'-monodeiodination during maturation provide further evidence for local rather than systemic regulation of this reaction.