The objective of this study is twofold: first, determining methods for setting parameters which describe thyroid gland responses at the initial stages of a given iodine-131 metabolic radiotherapy, and second, predicting treatment outcome. Radioactivity measurements were taken on patients suffering from hyperthyroidism. Two physicochemical parameters were analyzed and studied: iodine-131 thyroidal uptake and effective half-life. We thus achieved a much higher rate of success with minimal reverse results. The effective half-life, combined with thyroidal uptake, led to better predictions of success or failure for a given treatment. These set procedures allowed us to describe beforehand thyroid gland responses to metabolic radiotherapy as compared to the conventional methods exacting periods of more than six months.