An ultimate objective in the treatment of hyperthyroidism with radioactive I131 has been to develop a method of selecting for each patient a dose of the isotope which will destroy precisely enough thyroid tissue to produce the euthyroid state. The application of such a method would eradicate the therapeutic sequelae of radiation thyroiditis, thyroid storm, and myxedema. Protraction of the hyperthyroid state would not occur in patients who now require supplementary doses to correct for the inadequacy of the initial attempt. Although a search for a satisfactory method of dose prediction has been in progress since radioiodine was first used, in 1941, in the Massachusetts General Hospital,5 no ideal solution has yet been found. The thyroidal uptake of I131 can be measured by tracer doses of the isotope, but the mass of the thyroid gland can only be estimated with great error, either by palpation or by