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Several methods, giving values essentially in agreement, are now available to determine the iodine content-of plasma. Attempts to quantitate the iodine content of tissues have not yielded consistent results. The purpose of our paper is to report the application to this problem of a method based on isotopic equilibrium and using 1125 as a radioactive tracer. With this procedure, it is possible to measure the concentration of total iodine, iodide, thyroxine, triiodothyronine, and some unidentified iodine-containing substances in plasma, selected tissues, and excreta of the rat. METHODSDiets. Rats were fed two separate diets differing in their iodine content. The first, designated "high-iodine," consisted of a standard laboratory diet 1 containing 1.7 ,ug of iodine 2 per g of ration; the second, designated "low-iodine," was a special low-iodine ration 3 containing 0.06 /Ag of iodine 2 per g. To avoid the possibility of producing iodine deficiency (1), we added enough potassium iodide to raise the iodine content to 0.22 /Ag per g. The amount of iodine added to the diets as I"5 was negligible. The two diets, therefore, differed approximately tenfold in iodine concentration. Enough feed was obtained from the manufacturers to complete all the experiments with one lot, avoiding variations in the stable-iodine content.

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 In this connection, a previous report (4) that deiodination of L-thyroxine occurred in an athyreotic human has been retracted (5).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. METHODS Mlaterials antd preparationt. Samples of human kidney, liver, heart and skeletal muscle were obtained at operation. Adequate quantities of normal kidney tissue were available from nephrectomy in patients with renal carcinoma. WN'edge biopsies of liver were carried out during laparotomy. Intercostal or rectus muscle samples were obtained during thoracotomy or laparotomy, and limited quantities of left auricular appendage were available during mitral commissurotomy.The isolated tissue was immediately placed in chilled Krebs-Ringer phosphate solution. Slices (300 to 500 mg. wet weight) were prepared using a Stadie-Riggs apparatus.Reaction procedure. The reaction mixture (final volume 3 ml.) consisted of Krebs-Ringer phosphate solution (pH 7.4) to which 0.01 ,ug. of IP'-labeled L-thyrox-*

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