OFFICIAL L2= ONLY Radiciodine appearing in urine, except for that during a relatively short period following exposure, originates from the thyroid. In the calculation for urinary radioiodine it was aseumed that iodine is released from the : thyroid only as thyroxine, and that the release of thyroxine and its aubsequent degradation in the extrathyroidal hormonal space can be adequately described assumingfirst-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 asaumed 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. Rateg of urinary radioiodine excretion were calculated from the regult- 3.7 3.7 3.7 3.7 K day”! x 109 4.85 4.85 17.15 17,15 Rh * 0-60 LKT eK tA. (Ltn L-K The problem outlined above is a practical application of an analytical study on the uptake and excretion of iodine in man which will be described more fully in a UCRL report entitled, "The Dynamics of Iodine in Man." eT tK tantran L-jJ-k Iry * peak radioiodine content of the thyroid maaan iOuax } Xx = physical decay constant for 31 J = rate constant for the uptake of radioiodine in the thyroid vo. | On the basis of these calculations, the normal range of urinary radio- L = rate of turnover of extrathyroidal thyroxine K * rate constant for the release of iodine from the thyroid t = time. A sum:naryof the calculations made to determine the normal range of urinary radioiodine excretion at 15 days appears below. OFFICIAL USE ONLY ~) 0.050 0.086 0.16 0.21 iodine excretion at 15 days was estimated to be 0.05 to 0.2% of the peak thyroid content. Iu" radioiodine content of urine oal Y % of 1 jay 7.2 13.8 7.2 13.8 where ya ar, L. day! x10? ing expression shown below. diy aia J day”! see tS mm tenn a ee | LOuax CALCULATION OF URINARY RADIOIODINE EXCRETION Yook C, Ng woes TOEaTCRPEg Fe Urinary radiolodine excretion at 15 days. APPENDIX IEONRNR rs Bere = -T-