WITH the discovery of artificial radioactivity by Joliot and Curie (1), the building of the cyclotron by Lawrence and his associates (2, 3), and the development of the uranium chain reacting pile (4), the physicists have offered to biologists and physicians valuable tools for the study of intermediary metabolism. Indeed, the discovery of radioactive tracer techniques has been likened in importance (5, 6) to the invention of the microscope. The particular advantage of studies with these techniques lies in their applicability to the investigation of metabolic processes in physiologic equilibrium. As Marine (7, 8, 9, 10) and his associates demonstrated in 1915 and 1916, the thyroid gland is unique in that it has the ability to collect iodine selectively in relatively large quantities. It is, therefore, not surprising that studies with the radioactive isotopes of iodine have proved readily applicable and illuminating in the study of thyroid physiology.