The effect of alterations in the pituitary-thyroid axis on hypothalamic content and in vitro release of somatostatin-like immunoreactivity

The role of somatostatin (SRIF) in the regulation of thyroid homeostasis was studied by measuring hypothalamic SRIF-like immunoreactivity (SRIF-LI) content and in vitro release in hypothyroid (T3-treated and untreated), euthyroid, and T3-treated normal rats. Hypothalamic SRIF-LI content was decreased in hypothyroid rats and was restored to euthyroid levels by T3 treatment. SRIF-LI release from hypothalami of hypothyroid rats was decreased compared to that in euthyroid controls under basal and stimulated (60 mM K+ or 10(-6) M dopamine) in vitro conditions. The release of SRIF-LI from hypothalami of hypothyroid rats treated with T3 was restored to euthyroid levels. The release of SRIF-LI from normal rat hypothalami was unaffected by TRH or TSH but was stimulated by T3. These results suggest that T3 exerts its negative feedback effect on pituitary TSH release via stimulation of hypothalamic SRIF release as well as by a direct pituitary effect, and that the elevated TSH levels seen in primary hypothyroidism may result in part from a decrease in the tonic inhibitory effect of hypothalamic SRIF. Pancreatic, but not antral or colonic, SRIF-LI was increased in hypothyroid rats, and the levels were reduced by T3 treatment. The changes cannot be explained by alterations in food intake and may represent primary effects of the hypothyroid state.