In 1957, Astwood1reported on a patient with hyperthyroidism whose serum-bound iodine level was 4,200 meg. per 100 ml. A single dose of radioactive iodine led to a clinical remission of the hyperthyroid state but the bound iodine level in the serum remained astronomically elevated. Within a short period of time, a number of other patients presented themselves with serum-bound iodine values of 2,000 to 14,000 meg. per 100 ml. in the face of normal or increased radioactive iodine uptake and normal urinary iodine excretion. Of the 15 patients studied, 9 admitted previous exposure to iodine in one form or another. There remained, however, six patients who were unaware of any exposure to iodine, including radiopaque, diagnostic studies. An exhaustive chemical and spectroscopic analysis of the serum of the first patient finally led to the isolation of a compound which had the chemical formula of iophenoxic acid (Teridax). A