Calculation of Urinary Radioiodine Excretion Yook C. Ng Lawrence Radiation Laboratory, University of California Livermore, California Radioiodine appearing in urine, except for that during a relatively short period following exposure, originates from the thyroid. In the calcula- tion for urinary radioiodine it was assumed that iodine is released from the thyroid only as thyroxine, and that the release of thyroxine and its subsequent degradation in the extrathyroidal hormonal space can be adequately described assuming first order kinetics. Ranges for normal biological half-life of iodine in the thyroid and normal turnover rate of extrathyroidal thyroxine were selected from the best available data in the literature, The uptake of radioiodine was assumed to be exponential with a half-period of increase of 4.5 hours, and 60% of the iodine released when extrathyroidal thyroxine is degraded was assumed to be excreted in urine. Rates of urinary radioiodine excretion were calculated from the result- ing expression shown below. d 1, _ i 0. 60LKI_. - . _ e (K + N)t (L + A)t L- K - _ . e (J +K + NE. (L+A)t L-J- K Ly = radioiodine content of urine L = rate of turnover of extrathyroidal thyroxine K = rate constant for the release of iodine from the thyroid Iie = peak radioiodine content of the thyroid _ . 131 w = physical decay constant for I J = rate constant for the uptake of radioiodine in the thyroid t = time A summary of the calculations made to determine the normal range of urinary radioiodine excretion at 15 days appears below. b+