Metabolism of L-thyroxine by human tissue slices.

The deiodination of L-thyroxine to triiodothyronine has been observed in vitro using surviving rat kidney slices (1, 2). Surveys of other rat tissues have not shown consistent deiodination of tlhyroxine, but occasional appearances of trace quantities of triiodothyronine in surviving heart slices and diaphragm (3) have led to the belief that other tissues in the rat convert thyroxine to triiodothyronine at a slower rate than kidney. Relevance of these observations to human physiology might be questioned on the ground that conversion of L-thyroxine to triiodothyronine by extrathvroidal tissues is peculiar to the rat. In this connection, a previous report (4) that deiodination of L-thyroxine occurred in an athyreotic human has been retracted (5). It therefore seemed pertinent to examine the L-thyroxine deiodinating enzyme system of extrathyroidal tissues of the human. The purpose of this paper is to report suclh experiments employing tissue slices of human kidney, liver, skeletal muscle and heart.