Role of thyroxine in coordinate control of corticosterone and CBG in postnatal development

The role of thyroxine (T4) in the ontogeny of serum corticosteroid-binding globulin (CBG) and corticosterone has been studied in the rat. Daily injection of T4 (0.1 micrograms/g body wt) resulted in the precocious appearance of both CBG and corticosterone. A dose-response study revealed an increase in both CBG and corticosterone at thyroxine doses greater than 0.02 micrograms/g body wt. In propylthiouracil-induced hypothyroid pups, the developmental rise of both CBG and corticosterone was suppressed. To determine whether T4 elevation of CBG was mediated by corticosterone, serum CBG was measured in corticosterone-injected pups (10 micrograms/g body wt). No increase in CBG was detected on days 8 and 10. A slight increase, only one-eighth that elicited by T4, occurred on day 12. There was also no synergistic effect between T4 and corticosterone. We conclude that the postnatal increase in serum T4 concentrations cues the developmental rise of both CBG and corticosterone in the rat.