_7- - UCRL-12273 APPENDIX Calculation of Urinary Radioiodine Excretion . Yook C. Ne Radioiodine appearing in'urine, except for that during a relatively short périod following exposure, originates from the thyroid. In the calcula- tion for urinary radiolodine 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 dodine in the thyroid and norma], turnover rate of extrathyroidal thyroxine “were selected from the best available data in the literature. The uptake of radtolodine 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 radiolodine excretion were calculated from the result| ing expression shown below. “u at : : Ly \ ‘ go (K #a)t _ a7 (h +4 )t = Q-/60LKI_. vf L-K a (Tt K HA) oof tA)t — L-J-KX = radioiodine content of urine i z rate of turnover of extrathyroidal thyroxine K = rate constant for the release of godine from the thyroid Tip = peak radioiodine content of the thyroid A _J . 141 = physical decay constant for I «= rate constant for the uptake of radiolodine in the thyroid N - . + time 009538 | eerie Pe yey Vora, Ae ved