Circulating 131I thyroxine and thyroid cancer.

Abstract

We have developed a blood test to detect the presence of thyroactive tissues in patients being evaluated for ablative therapy of differentiated thyroid carcinomas. This test is based on the assumptions that if any thyroactive tissue is present, some radioactive T4 will be manufactured after administration of radioiodide, and that some of it will be released into the circulation. Twenty serum samples obtained from 19 patients at the time of total body 131I scanning to evaluate for the need of further 131I therapy were analyzed chromatographically, together with an internal 125I T4 marker, for the presence of 131I-labeled T4. Three patients also were studied 7 days after a treatment dose of 131I, when they returned for posttreatment scan. The chromatographic method used (Sep-Pak followed by HPLC) removes all radioiodide from the T4 fraction, so the results were interpreted as negative or positive based upon the absence or presence of detectable counts in the T4 fraction. Twelve patients (16 studies) with positive total body 131I scans all demonstrated circulating 131I T4. Seven patients with negative whole body scans were all negative for 131I T4. On the other hand, 2 patients with positive thyroglobulin assays had negative total body scans and 131I T4 tests. Both of them had independent evidence of the presence of thyroid cancer that did not concentrate radioiodine. Three patients had positive scans and 131I T4 tests but negative thyroglobulin assays. We conclude that measurement of circulating radioactive T4 is a sensitive test for the presence of normal or malignant thyroactive tissues which are likely to respond to 131I ablation.