METABOLISM OF l-THYROXINE BY HUMAN TISSUE SLICES *

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Abstract
The enzymatic deiodination of I131-iabeled L-thyroxine to triiodothyronine by human extrathyroidal tissues was examined by using surviving slices of kidney, heart, liver and skeletal muscle. With kidney, the conversion of thyroxine to triiodothyronine ranged from 11 to 38% in 6 hr. With heart, trace amounts of triiodothyronine were noted occasionally, but in quantity insufficient to establish definitely the reaction in this tissue. Triiodothyronine formation was not observed in liver and muscle. The greater activity of the thyroxine deiodinating enzyme in kidney is presumably related to the high metabolic rate of this tissue.