Our accumulated evidence has shown that triiodothyronine.is not the fesult cf irradiation: first, because it appeared very early before appreciable radjaticn was delivered to the gland; and second, because it did not increase fimther wi’ ime but was subsequently greatly overshadowed by thyroxine after hours later wifen the ‘ radiation effect should have become more significant. Finally, it has bee easier to relate the preserce of triiodothyronine to high degrees of clinical toxdcity than tc subsequent clinical -herapeutic success of a single treatment dose of 131y In order to get a better perspective of the triiodothyronine metabolism in patients treated with nadioactive loding considerable time was spent studying the fate of triiodothyronine labeled with ty and administered to humans. 2 were made in patients with Graves' disease before and after treatment withp as in normal individuals. There was special interest in those who were origi known to produce appreciable levels of triiodothyronine. tions i, as sei The fate of oiidiotnyrenize was observed in these individuals by quantitative serial chromatograms on samples of serum and urine. In addition, the radioactivity was measured over the live accumulated in this organ. as it During the firat 4% hours following the administretion of labeled triiodothyronine, more of the material remained in the circulation patients with Graves' disease than in normal controls. At the same time more radicaktivity ws excreted in the uringof Graves' disease than in the normals. This urinary] increase was largely due to +31l1-iodide. The increased excretion in the patients wi Graves’ disease occurred regardless of the metabolic state--whether it be untreated] hyperthyroidism or hypothyroidism following treatment. Measurements over the liver revealed a maximal concentration minutes after the intravenous injection of triiodothyre : This was followed by a marked fall. In Graves' disease there was a seconda cyclic of radioactivity in the liver but this did not occur in the normal confrols. Te ate of the triiodothyrenine in the peripheral tissues was not studied. ihe fact that noaicdethyrenine levels in the serum of Graves’ disease remained higher|than that in normal individuals during the first 8 hours seemed to conflict with the @bservatim t more radioactivity appeared in the urine as iodide. The only availabl@ explanation that the triiodothyronine which was removed from the circulation in Grayes" disease deiodinated more rapidly than in the normal subjects. It was fiorvther ngtec that iiodothyronine was retained in larger amounts and for a longer time in thd liver ct lightly more triiodothyronine excreted through the kidneys in the severely : ividual than in the post-treatment hypothyroid Graves' disease, the diffe ess impressive than the difference between all patients with Graves' diseage az These observations shed some light on the difference between patiants with ves' disease and normal subjects with respect to triiodothyronine but did nct ntribute information to an understanding of any possible relationship betwee liodothyronine metabolism and the radiation effect on the hyperfunctioning gland. eS, I.B., and Dobyns, 3.M.: The Metabolism of Triiodothyronine in Graves’] disease. - Clin. Endocrinol. & Metab. 20: 68-80, January, 1960. 1