Thyroxine: Conversion to Triiodothyronine by Isolated Perfused Rat Heart

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Abstract
Thyroxine labeled with carbon-14 and iodine-125 was perfused through surviving rat hearts. Only when unlabeled triiodothyronine was added as a carrier could the newly formed doubly labeled triiodothyronine be isolated. The fact that this triiodothyronine was labeled with the correct ratio of carbon-14 to iodine-125 indicated that it originated from thyroxine. Approximately 5 percent of the initial carbon-14 radioactivity was found in the recovered triiodothyronine.