Alteration of thyroidal responsiveness to TSH under the influence of circulating thyroid hormone: short feed-back regulatory effect.

In order to examine the hypothesis that the thyroidal responsiveness to TSH is under the influence of thyroid hormone, following the T3 injection to the mice, serum T3 concentrations and the response of thyroid tissue to a fixed dose of TSH in terms of intracellular colloid droplet formation was studied. The colloid droplets induced by TSH was significantly reduced when serum T3 was decreasing, while it was significantly increased when serum T3 was increasing. This results demonstrate for the first time the existence of short feed-back loop regulating intra-thyroidal function by circulating T3. To delineate the possible mechanism of action of T3, the thyroid gland of mouse whose serum T3 concentration was elevated by injecting 50 mug T3, was incubated with TSH in vitro. TSH-induced cyclic AMP generation was not inhibited at all but colloid droplet formation was significantly inhibited in the thyroid tissue of the animal whose serum T3 concentration was enormously high. Thus, it was demonstrated that the site at which T3 affects is beyond cylcic AMP generation but prior to endocytosis, being consistent with our previous results.