Effect of increased circulating thyroid-stimulating hormone on in vitro thyroid-stimulating hormone stimulation of...

Hormones have been shown to regulate the number and/or binding properties of their own receptors. The present studies examined the effect of chronic increased endogenous TSH levels, induced by tapazole or thyroidectomy, on in vitro TSH responsiveness and binding in thyroid and adipose tissues. The results showed that TSH and prostaglandin E1 significantly increased cAMP levels in the thyroids of weight- and age-matched controls, whereas thyroids from tapazole-treated rats responded only to prostaglandin E1. Iodide organification was also measured, and the thyroids from tapazole-treated rats showed a significantly reduced effect of TSH compared to weight- and age-matched controls, although stimulation by dibutyryl cAMP was equivalent in all three groups. TSH or epinephrine stimulation of cAMP and glucose oxidation was equivalent in adipose tissue from control and hypothyroid rats. There was a significant 50% reduction in the number TSH-binding sites in thyroids from tapazole-treated rats: the affinity remained unchanged. [125I]TSH binding to adipose tissue plasma membranes was similar in control and hypothyroid groups. These studies demonstrate that elevated levels of TSH appear to regulate the number of TSH receptors in thyroid, but not adipose, tissue.