THE DIPHASIC CHARACTER OF THYROIDAL I131CLEARANCE*

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Abstract
Direct measurements of thyroidal, serum and urinary radioactivity were made in normal individuals 0.5, 1, 2, 3, 6, 12, 24, 48, 72, and 96 hours after the intravenous administration of 200 microcuries of I131. From these data the thyroidal I clearance for each time interval was calculated. In 13 of 15 normal individuals, the clearance was diphasic. The 1st phase lasted for approximately 3 hours with a mean clearance value of 1 I/hour. During the 2d phase, the mean clearance value was 0.6 I/hour. The change in the thyroidal I131 clearance coincided with alterations in both the disappearance rate from blood and the urinary excretion rates. It is suggested that during the 1st phase, the major reaction is the equilibration of thyroidal cellular radioiodide with serum radioiodide, whereas during the 2d phase the limiting factor becomes the "rate of hormonal synthesis.