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November 23, 1970

T3 Thyrotoxicosis and the 24-Hour Uptake of Radioactive Iodine

JAMA. 1970;214(8):1566. doi:10.1001/jama.1970.03180080146038

To the Editor.—  Dr. Kenneth Sterling and his collaborators (213:571, 1970) draw attention to the fact that the 24-hour thyroidal radioactive iodine (131I) uptake is normal in many patients with thyrotoxicosis caused by increased production of triiodothyronine (T3) rather than thyroxine (T4). They justly remark that "this would be compatible with formation of T3, which requires less iodine than T4." The iodine content of T3 is only three-quarters that of T4, and its potency about twice as high per unit of weight. The authors add, however, that "it is advisable to employ screening with the T3 suppression test, regardless of the initial 24-hour thyroidal radioiodine uptake value." I don't think this is generally necessary.Very probably, the relatively increased production of T3 only partially explains the low-normal 24-hour uptakes. Values of 8% to 18%, found in six out of 25 cases