Concept and validation of a simple model of the intrathyroidal iodine kinetics

Aim of this study is the introduction and validation of a simple model of the intrathyroidal iodine kinetics, designed for optimizing radioiodine therapy planning and dose measurement in a routine clinical setting. Methods: The new model defines the intrathyroidal iodine kinetics as balance of the thyroidal iodine intak and -excretion, characterized by the two exponential equations At=A0* (1-exp(-λ1t)) and At=A0* (exp(-λ2t)-1), respectively. A0 describes the theoreticaly maximum iodine uptake when the thyroidal iodine excretion is ignored, λ1 and λ2 represent the constants characterizing the iodine intake and excretion, respectively. The thyroidal iodine content at the time t equals the sum of both functions, which is At=A0* (exp(-λ2t)-exp(-λ1t)). In 25 patients with autonomous goiter / nodules (n=18), Graves' disease (n=5), or endemic euthyroid goiter (n=2), the iodine uptake in the thyroid during the radioiodine therapy as fraction of the applied activity was determined daily, with the remaining body covered by a lead shield. On average, 7.2 measurements were performed per patient (minimum 4, maximum 13). With these uptake values, individual regression curves were fitted using the above equation, and the difference between the actual measurements and the corresponding values of the regression curves was determined. Results: The average deviation of the 179 uptake values frorm the calculated points of the respective regression curves was only 1.4%. There was no significant difference between the three disease groups. The distribution of the relative deviations during the individual courses was constant, systematic errors were not detected. Conclusion: Our results suggest that the intrathyroidal iodine kinetics can be precisely described with the model At=A0* (exp(-λ2t)-exp(-λ1t)). With only three measurements, the trend of the curve can be calculated, which allows to determine the total radioiodine storage in the thyroid. (orig.)