Effects of dexamethasone on the hypothalamic-pituitary-thyroid axis in rats

Effects of dexamethasone on the hypothalamic-pituitary-thyroid axis in rats were studied. Rats given saline (group A), 25 micrograms of dexamethasone/100 g body weight (group B) or 500 micrograms o dexamethasone/100 g body weight (group C) were serially decapitated and brain tissues and blood were obtained. TRH contents in the hypothalamus, plasma concentrations of TRH, TSH, T4, T3 and reverse T3 were measured by specific radioimmunoassay. TRH contents in the hypothalamus were significantly increased at 3 h in group B after dexamethasone treatment. In group C, however, they significantly increased for 1 to 3 h, but then decreased with the minimum at 20 h. TRH plasma levels significantly increased with the maximum at 4 h in group B and at 24 h in group C. TSH plasma levels were significantly decreased initially and then significantly increased with the maximum at 5 h in group B and at 24 h in group C. TSH responsiveness to TRH was significantly decreased at 1 to 3 h in group B, but in group C it was significantly decreased initially and the significantly increased. T3 plasma levels significantly decreased at 1 to 3 h in group B and at 1 to 18 h in group C. Reverse T3 plasma levels were significantly increased with the maximum at 3 h in group B and at 6 h in group C. The results demonstrate that dexamethasone may act on multiple sites of the hypothalamic-pituitary-thyroid axis in rats and that its effect depends on the dose used or the time interval after dexamethasone treatment.