Iodine Intake, Excretion and Thyroidal Accumulation in Healthy Subjects1

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Abstract
This inpatient iodine balance study tested and verified the hypothesis that iodine excretion is linearly proportional to iodine intake within the physiologic range of the latter; and the research also identified an excretion of iodine independent of intake (obligatory excretion), presumably from endogenous sources. Each of 3 healthy euthyroid volunteers was fed 3 levels of iodine intake ranging from 27.7 to 606.5 μg I/day in random order for 54 days—18 days at each intake level. Urine and feces were completely collected and their iodine content was determined chemically. The PBI and PII (by the creatinine method) were determined on 5 days during each 18-day period. At the end of each 18-day period, plasma and urinary radioactivity and the relative thyroidal I uptake (U8) were determined with 132I. From these data the absolute iodine uptake (AIU), thyroidal (TCL) and renal (RCL) iodide clearances were computed. The subjects were in negative iodine balance at the end of each 18-day period and they were also in negative balance during 91% of the 159 days studied. The results demonstrated an obligatory excretion of I amounting to 57 μg I/day which was independent of I intake; and it was also shown that the fraction of intake excreted was approximately 1 (i.e., 100%), so that the iodide compartment appeared to be in a steady state.

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