Estimation of total body iodine content in normal young men

Total body iodine content was estimated in six normal young men, who underwent I-125 balance studies during 64-92 days of daily I-125 administration. Total body retention of I-125 was measured as the difference between total administered I-125 and that collected in the urine and feces. Extrathyroidal I-125 was the difference between total and thyroidal I-125 content. The time-activity data for the ratio of extrathyroidal to total retained I-125 were fitted to a growth (inverse exponential) function. Fits of this growth function to the individual data sets yielded asymptotes, the equilibrium extrathyroidal/total I-125 ratios. The slopes of this function predicted the time that would have been required to achieve I-125/I-127 equilibration (approximately 10 months). Geometric mean for the asymptotic extrathyroidal/total I-125 ratio was 0.34 (range, 0.19-0.63), if it was assumed that measured urine and feces represented all of the I-125 lost to the body. If 90% measurement of I-125 loss was assumed, the geometric mean ratio was 0.32 (range, 0.17-0.60). Assuming that 90% of total loss is reflected in measured excreta and that total iodine content of the thyroid gland is 10 mg, geometric mean for total body iodine in these subjects was 14.6 mg (range, 12.1-25.3 mg).