IODINE-CONTAINING COMPOUNDS OF EXTRATHYROIDAL TISSUES*

Several methods, giving values essentially in agreement, are now available to determine the iodine content-of plasma. Attempts to quanti- tate 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. METHODS Diets. 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 po- tassium 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.

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