ar repeated doses were required, the uptake was usually progressively 1 ss and the retention of the isotope more sustained. As has been pointed out i previous re- raorts, some of the data collected in the past represents some of the ost complete observations cn individual patients available. £ the National Thyrotoxicosis Follow-Up Study. collected These became a2 part In addition to enormous amounts of chemical data over several months on each of these vatients, there has been continu- ous long-term follow-up of physiological function of the radiated g land. A knowledge of the precise rad dose received by the gland is the only way that the outcome of radioiodine therapy can be evaluated. Unfortunately, the dose that was administered to the patient is no measure of the amount of radiation received wi hout detailed study In a few cases, histologic samples of the gland have become available because surgery was indicated owing to the appearance of a mass or persistance of hyperthyroidism. The procurement of tissue has provided an opportunity to study by auto- radiography the effects of the radiation in various areas of the g and. Thus, we have continued to follow our radioiodine therapy cases diligently | vith repeated examinations to look for the development of masses and to reapprais > thyroid function. Some of the significant biochemical, radiological, and clinic hl correlations that have been found are: If the serial quantitative chromatograms on the blood fdllowing a therapeutic dose of 131; show radioiodinated mono- and diiodotyros ine after a few days, a precipitous decline in radioactivity over the thyroid usually occurs and this is followed by extensive destruction of the gland. Chromatograph- ically insoluble iodinated polypeptides also appear in the b lood. indicates gross destruction of the gland with the releas Material into the circulation. This of colloid