Effect of dexamethasone on the secretion of thyrotropin in the rat: dose and time relations

Serum TSH and corticosterone concentrations were measured in intact rats and in rats given TRH or exposed to short-term cold 3 h and 12 h after pretreatment with dexamethasone in various doses. Dexamethasone given 3 h before experiemtns significantly depressed both TRH- and cold-induced TSH responses at all dose levels. Dose of 25 pg/100 g body weight significantly depressed serum TSH concentration when given 3 h before the experiment. However, when given 12 h before the experiment the drug augmented TRH-induced TSH secretion, although the cold response was unaltered. In intact rats dexamethasone significantly depressed serum TSH concentration in doses of 250 and 500 mug/100 g body weight. In all experiments the steroid blocked ACTH secretion. These results support the view that the effect of dexamethasone on thyroid function is highly dependent on the time relations. A single large dose of dexamethasone has first an inhibitory effect at the pituitary level and then facilitates pituitary to TRH and at the same time inhibits secretion of TRH in response to cold.