The formation of thyroid hormones in vivo after the administration of K131I and thyrotropic hormone

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Abstract
The formation of thyroid hormones in vivo has been studied in the blood of different animals with the aid of a new micromethod. After an injection of radio active iodine into rats, it was found that the radioactivity of triiodo-thyronine initially exceeded that of thyroxine, later becoming much less. It is thought that triiodothyronine is a precursor of thyroxine. Six hours after the injection of thyrotropic hormone, together with radioactive iodine, the radioactivity of the blood thyroxine was much higher, and that of triiodothyronine much lower, than that of the controls. Thyrotropic hormone has an accelerating effect on the transformation of triiodothyronine into thyroxine. After an injection of thyrotropic hormone into rabbits, the protein-bound iodine of the blood increased. Second or third injections at intervals of 10-18 days produced a decrease.