Criteria for the administration of KI for thyroid blocking of radioiodine

Scientific data are reviewed to evaluate the risks of radioiodine uptake and to compare those risks with the benefits and risks of low milligram doses of stable potassium iodide (KI). The limit of 25 rad to the thyroid due to radioiodine uptake is adopted as the "break-even" point above which 130-mg KI doses should be administered. The biological and radiological kinetics of radioiodine for protracted uptakes were derived from the Medical Internal Radiation Dose Committee (MIRD) model (MIRD75). Resulting calculations yielded estimates of dose commitment rates to the thyroid as a function of thyroidal uptake. The extrapolated value of the 1-hr inhalation curve for 131I with 30% uptake compares well with the established MPCa value and intercepts the origin. The calculated KI-blocking efficiency as a function of time after radioiodine uptake agrees well with previously reported experimental data. The prevention or "blocking" of 25 rad to the thyroid was the criterion used to define critical values of radioiodine in the thyroid. Critical values are functions of isotope, the duration of uptake and the elapsed time between inhalation and assay of thyroid content. The presence of radioiodine in the thyroid in amounts greater than the critical value indicates that more than 25 rad to the thyroid can be averted, and KI should be administered in the absence of contraindications. Critical average concentrations are implicitly defined by the method of calculation used in the derivation. Critical average concentrations are presented as criteria for KI administration when assays of the radioiodine content of the thyroid are unavailable. Illustrative applications of critical values and critical average concentrations are presented in the Appendix.