THE METABOLISM OF THYROTROPHIC HORMONE IN THE RAT1

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Abstract
Quantitative estimation of endogenous and exogenous thyrotropic hormone in the blood of normal, goitrous, and hypophysectomized rats was made by means of the stasis tadpole technique of bioassy. Endogenous thyroid stimulating hormone (TSH) blood titers were found to be significantly elevated in rats chronically treated with propylthiouracil. The rise did not parallel the thyroid hyperplasia. Mean per cent recoveries of a single dose of intravenously administered TSH from the circulation of goitrous rats were consistently greater than normal. The slow rate of disappearance of exogenous TSH from the blood of hypophysectomized rats could be accelerated by previous elevation of the metabolism with calorigenic agents. Thyroxine and triiodothyronine were much more effective than dinitrophenol in hastening the removal of thyrotropic hormone from the circulation. The persistence of exogenous TSH in the blood of rats with either hyperplastic or atropic thyroids does not support the hypothesis that the thyroid gland plays a major role in the inactivation of circulating hormone. The concept is offered that the removal of thyrotropin from the blood is primarily influenced by metabolic levels in tissues responsive to the thyroid hormone.