Influence of Thiourea, Potassium Perchlorate and Thiocyanate and of Graded Doses of Propylthiouracil on Thyroid Hormone Metabolism in Thyroidectomized Rats, Isotopically Equilibrated with Varying Doses of Exogenous Hormone

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Abstract
Daily doses of propylthiouracil (PTU) as low as 50 μg per rat result in a depression of the peripheral deiodination of Lthyroxine in thyroidectomized animals isotopically equilibrated with I131-labeled L-thyroxine. Administration of PTU is accompanied by similar alterations of the peripheral metabolism of triiodo-L-thyronine in thyroidectomized rats maintained with the latter hormone. Maximal extrathyroidal effects of PTU on the metabolism and effectiveness of L-thyroxine are obtained with doses of 500 ng per rat or higher. The influence of a given dose of PTU was studied in groups of thyroidectomized rats which were equilibrated isotopically with varying doses of L-thyroxine ranging from 0.5 to 50 μg per rat per day, and it was observed that the drug depressed thyroxine deiodination to a rather constant proportion of the available hormone. Therefore the effects of PTU on the actual amount deiodinated peripherally may be overcome by adequately increasing the hormonal supply. Thiourea, potassium thiocyanate and perchlorate do not depress the peripheral deiodination of thyroid hormones. The bearing of these results on the present ideas regarding thyroid-pituitary interrelationships is discussed.

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