Effect of endogenous TSH on the in vitro responsiveness and binding of TSH in rat thyroid tissue

These studies examined the effect of deficient endogenous TSH (hypophysectomy) on in vitro TSH responsiveness and binding in rat thyroid tissue. The results showed that thyroid from hypophysectomized rats were hyper-responsive to stimulation of cyclic AMP by TSH while PGE1 stimulation of cyclic AMP was similar in hypophysectomized rats compared to controls. When iodide organification was measured, the TSH and dibutyryl cyclic AMP stimulation in hypophysectomized rats was equivalent to the response of control animals. Binding studies demonstrated there was an increased number of TSH receptors in the thyroid from hypophysectomized rats compared to the control group. The affinity constant was unchanged. The results demonstrate that endogenous TSH appears to inversely regulate the responsiveness and number of TSH receptors in thyroid tissue. The effect of an i.v. injection of TSH on the levels of cyclic AMP and binding characteristics of TSH was also examined. Although cyclic AMP levels were significantly increased after a 10 min in vitro incubation of the lobes from TSH-injected rats as an indication of persistently bound TSH, the number of TSH binding sites was the same in the saline the TSH-injected animals.