radioisotopes of iodine, especially iodine-124 and todine-130. BIOLOGICAL BEHAVIOR Technetium-99m has been utilized to study the trapping function of the thyroid, this being the rate Limiting process in hyperthyroidism. Previous studies have shown that the 20 minute technetium uptake is as diagnostically reliable as the 24 hour iodine uptake in assessment of the functional status of the thyroid (8). Another advantage of radio- pertechnetate is the ability to evaluate functional status while the patient is on thyroid blocking agents. Diagnosis of hypothyroidism is no more accurate with technetium than with radioiodine. The uptake of technetium is affected by exogenous iodine, thyroid hormones and thyroid stimulating hormone in the same way as radioiodine uptake (8,19). The usual range of uptake of pertechnetate ig rather low, on the average about 1-2%. This results in high levels of background activity, making determination of uptake somewhat more difficult. Recently computer techniques have been used which aid in this determination (9,20). Radioiodine can also be used to evaluate trapping. However, binding occurs very rapidly so that in order to completely separate out the binding phase it would be necessary to use a binding blocking agent. Iodine-131 has net been entirely successful for this because of the high radiation dose ircurred from the use of sufficient radioactivity to effectively determine early uptakes of small proportions of the administered activity. ‘ CLINICAL UTILIZATION The determination of technetium-99m uptake by the thyroid is a convenient, one visit procedure, (9). Usually the 20 minute uptake is determined This may not be the maximal uptake during the first half-hour but 5001396