Estimation of Total Body Iodine Content in Normal Young Men

Abstract

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