In 1936, Karl Compton, then president of the Massachusetts Institute of Technology (MIT) and the thyroid group of the Massachusetts General Hospital (MGH), undertook a joint study that led to the production of small amounts of short-lived radioiodine (iodine 128, half-life, 25 min). The original intent was to use it for diagnosis and treatment of thyroid disease, but in order to explore the underlying physiology, their first work was performed in rabbits and published in 1938. It clearly showed that the radioiodine was selectively and avidly taken up by the thyroid gland. It was immediately apparent to the MGH-MIT group and another team working at the Berkeley, CA cyclotron that longer-lasting iodine isotopes were needed, and soon both developed procedures for cyclotron-produced 130 (half-life, 12.5 hr) and 131I (half-life, 8 d). In 1939, the Berkeley group, using 131I, was the first to show that the normal human thyroid gland accumulated radioiodine. By 1941, the MGH-MIT team, using mainly 130I, was able to successfully treat a few patients with hyperthyroidism, and so achieved their original goal. The Berkeley group did the same a few months later, using mainly 131I. Both presented results at the same meeting of the American Society for Clinical Investigation in Atlantic City, NJ in the spring of 1942. This was in the midst of World War II and it was not easy to get much 130I or 131I, so experience was limited. Although effective, radioiodine treatment of hyperthyroidism had not been widely adopted by the end of the war in 1945, partly because radioiodine remained in short supply and partly because another medical therapy for hyperthyroidism, antithyroid drugs, had been invented. However, by 1946, fission-derived radioiodine became readily available as a by-product of the Manhattan project in Oak Ridge, TN; hundreds of patients were treated within a few years, both for hyperthyroidism and for thyroid cancer. A new treatment, based on the physiological application of a radioisotope of iodine, was then a reality.