THE NATURE OF THE SERUM IODINE AFTER LARGE DOSES OF I131*†

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Abstract
The serum radioiodine was studied in 11 patients following "tracer" doses of I131. More than 90% of the serum radioiodine was precipitable with the Somogyi reagents, and 70-89% was extractable with n-butanol. In 10 of 16 patients who were studied after "therapeutic" doses of I131, it was found that although the serum radioiodine was largely precipitable, only from 12-66% was butanol-soluble. After alkaline hydrolysis, the butanol-insoluble radioiodine fraction was shown to release thyroxine, diiodotyrosine, and iodide in proportions comparable to those found in thyroglobulin. The occurrence of this abnormal material in the blood appeared to be related to the amt. of radiation delivered to the thyroid tissue, and was most pronounced in those patients who had the most severe radiation damage. In some patients, the presence of butanol-insoluble radioiodine in the serum was accompanied by a rise in serum protein-bound stable I. This material did not appear to produce a calorigenic effect. The serum radioiodine elaborated by functioning carcinoma of the thyroid appears to be identical with thyroxine.