functioning or hypofunctioning relative to surrounding thyroid tissue on the radioiodine scan. observers (11-16). This has also been noted by a number of other The clinical diagnoses have varied in these patients and the clinical significance is not settled at this time. We have, more recently, been carrying out two further studies of 1fodine-123 utilizing the intravenous route. These studies were initiated to determine the most appropriate time for imaging with iodine-123, to see if the standard apparatus for radioiodine uptakes is applicable to iodine-123 and to evaluate iodine-123 in the early phase of uptake, the “trapping phase" and compare its behavior with technetium. This last study is still being carried out. In a now completed study (24) we determined thyroid uptake of intravenously administered iodine-123 and orally administered iodine-131 at 2, 6 and 24 hours. At 2 and 6 hours the percent uptake of the intravenous dose was slightly higher than the oral dose although not of statistical significance. The uptake at 24 hours was approximately the same for both routes (Fig. 5). The increase in the fraction of the dose in the thyroid at 24 hours over that in the gland at 6 hours was about 50 - 60%. hardly compensates for radioactive decay. This Assuming 15% uptake at 6 hours and 22% at 24 hours after a lOO0uCi dose, the level of radioactivity in the gland at 6 hours would be 1lpci and at 24 hours, 6,Ci. Therefore, imaging at 6 hours could be performed with one-half the administered activity required for 24 hour imaging. The study also indicated no advantage to the intravenous route of administration. A number of investigators have studied the early uptake of radioiodine after intravenous administration (25-29), The difficulties in quantitating these uptakes are the same as those encountered with technetium-99n, 5001358