Hormonal control of postnatal development of corticosteroid-binding globulin.

The hormonal regulation of the ontogenic rise in serum corticosteroid-binding globulin (CBG) has been studied in the rat. Pups received daily injections of estrogens (either estradiol, estrone, or diethylstilbesterol, each at 0.05 microgram/g body wt) or thyroxine (0.1 microgram/g body wt) on postnatal days 2-7. When CBG binding capacity was determined on day 8, only the thyroxine-injected pups were found to have elevated CBG. This effect of thyroxine on CBG binding capacity was further studied by daily administration of the hormone (0.1 microgram.g body wt-1.day-1) between postnatal days 5-12. This caused a precocious rise in serum CBG, with CBG values 6- to 38-fold higher than euthyroid controls on days 8, 10, and 12. Conversely, in pups made hypothyroid by administration of propylthiouracil, the normal ontogenic increase in CBG was suppressed. Thyroxine replacement resulted in the reappearance of the CBG rise. These results suggest that the developmental rise in the binding capacity of CBG is independent of estradiol and estrone and instead is elicited by the rising concentrations of circulating thyroxine that normally occur in the early postnatal period.