ANALYSIS OF THE ACTION OF PROPYLTHIOURACIL ON THE PITUITARY-THYROID AXIS OF RATS1

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Abstract
Propylthiouracil (PTU) augmented the thyroid cell height response in hypophysectomized rats receiving graded doses of thyrotropin (TSH), without changing the slope of the log dose TSH-response curve. It enhanced the effectiveness of the hormone on the thyroid:serum iodide concentration (T/S) ratios of the same animals, altering the character of the curve obtained by plotting the response against the log dose of TSH. The goitrogen also elevated the T/S ratio of hypophysectomized rats receiving TSH. It is concluded that the hyperplastic changes of the thyrpid which are evoked by goitrogen treatment are due to at least 2 factors: increased release of TSH, and augmentation of its action. In the causation of the concomitant rise of the T/S ratio 2 further factors seem to be involved: a direct action of the drug on the thyroid, and an apparent selective potentiation of the effect of TSH on the thyroidal iodide concentrating mechanism. It is suggested that PTU exerts the latter 2 influences by decreasing the thyroidal concentration of a hypothetical inhibitor of the iodide concentrating mechanism, possibly thyroid hormone itself.