Control of the hypothalamus-pituitary-adrenal axis in young and older rats injected with thyroxine.

Administration of T4 on alternate weeks for 30 weeks at a dosage which does not alter body weight depresses basal serum corticosterone levels in older rats (575 days), but not in young animals (260 days). Similar serum corticosterone response to HPA axis stimulation occurs regardless of age or T4 injection.