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Background: In the current clinical practice, radiometabolic therapy of Graves' disease is based on the administration of an amount of 131I-iodide defined according to two different modalities: administration of a fixed activity (up to a maximum of 555 MBq allowed by many national legislations on an outpatient basis) or an 131I-iodide activity individually defined so as to reach the certain fixed values of target thyroid-absorbed dose. The aim of this study was to prove the effectiveness of a personalized approach to cure Graves' disease based on the administration of a 131I activity defined according to the desired "optimal final thyroid mass." According to this model, one can predict the thyroid-absorbed dose following the administration of an activity as a function of the desired reduction of the baseline, pre-treatment mass of the gland. Methods: A total of 217 patients with Graves' disease were included in this study. They were randomly divided into five groups, of which four groups were divided based on fixed thyroid-absorbed dose value of 100 (Group A, n = 29), 200 (Group B, n = 25), 300 (Group C, n = 51), and 400 (Group D, n = 25), respectively, whereas patients of the final thyroid mass (mf) (Group V, n = 87) received a 131I activity calculated based on the desired "optimal" final thyroid mass according to the so-called "volume algorithm" approach. Results: At the 1-year follow-up, therapeutic success (i.e., a state of either hypothyroidism or euthyroidism) was achieved in 48% of patients in Group A (100 Gy), 64% in Group B (200 Gy), 78% in Group C (300 Gy), 96% in Group D (400 Gy), and 92% in Group V (mf). A significantly higher proportion of patients were was cured in Groups D and V (mf) than in Groups A, B, and C (P < 0.01). There was no statistical difference in cure rates between Groups D and V (mf). The median thyroid-absorbed dose was 407 Gy for Group D (400 Gy), significantly higher than 296 Gy for Group V (mf) (P < 0.001) and also significantly greater than the administered 131I activity (494 vs. 345 MBq, P < 0.001). Conclusions: This study indicates that the most effective thyroid-absorbed dose to be delivered for the radiometabolic therapy of patients with Graves' disease should not be based on a fixed dose but rather should be personalized. The new method based on thyroid mass reduction allows the optimization of 131I-iodide therapy for Graves' disease on an individual basis, achieving a high therapeutic efficacy while at the same time avoiding the administration of unjustified greater activities of 131I. [ABSTRACT FROM AUTHOR]