Abstract
The administration of propylthiouracil to rats is followed by a prompt increase in circulating TSH (1,2). To test further the idea that this is related to the extrathyroidal effects of PTU on l-thyroxine (l-T4), we have screened the effects of several other antithyroid drugs on TSH release. We were especially interested in those drugs, such as thiourea and methylmercaptoimidazole (MMI), known to lack extrathyroidal effects on l-T4 metabolism. The change occurring after a single injection of the drug in the rate of thyroidal 131I release of rats on ClO4− was taken as an indirect measure of a possible change in circulating TSH. Contrary to what we expected on the basis of the idea indicated above, we found that MMI behaved similarly to PTU, inasmuch as it promptly discharged 131I from the ClO4−-blocked thyroid. To see whether this similarity might be only apparent, the experiment was repeated in hypophysectomized rats on ClO4− and exogenous TSH. It was found that under such conditions PTU had no effect, whe...
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