Divergent Changes of Thyroxine-5′-Monodeiodination in Rat Pituitary and Liver during Maturation*

Abstract

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. (Endocrinology106: 1405, 1980)