The influence of dexamethasone on serum thyrotrophin and thyrotrophin synthesis in the rat.

Glucocorticoid hormones suppress circulating concentrations of thyrotrophin (TSH), but their effect on synthesis of TSH in the pituitary gland is unclear. We have examined the influence of the glucocorticoid dexamethasone on serum TSH, pituitary TSH content and TSH beta- and alpha-subunit mRNA concentrations in pituitary cytoplasm in both the euthyroid and hypothyroid rat, and following triiodothyronine (T3) treatment in the hypothyroid rat. The rise in serum TSH in hypothyroidism was attenuated in animals treated with dexamethasone; in addition the suppression of serum TSH 6 h after T3 administration to hypothyroid rats was enhanced by dexamethasone. In contrast to the changes in serum TSH, pituitary TSH content was unaffected by dexamethasone. Furthermore dexamethasone had no significant effect on changes in pituitary cytoplasmic TSH beta- and alpha-subunit mRNA levels with thyroid status. These findings demonstrate that dexamethasone exerts differential effects on serum TSH levels and TSH biosynthesis which contrast with those of thyroid hormones.