Evaluation of Radioiodine (I-131) Therapy Success in Different Etiological Forms of Hyperthyroidism and Analysis of Response According to Administered Dose.

INTRODUCTION: Radioactive iodine (I-131) has been used for decades in the treatment of hyperthyroidism. However, the final outcome depends not only on the underlying etiology but also, to a large extent, on the initial administered activity. OBJECTIVE: The aim of this study was to determine the success rates of radioiodine therapy in Graves' disease, toxic multinodular goiter (Plummer's disease), and toxic adenoma, and to precisely assess the impact of different I-131 activities on the risk of treatment failure. METHODS: The study included 146 patients with a clearly established etiology of hyperthyroidism. Each patient received one of four fixed activities: 10 mCi, 15 mCi, 20 mCi, or 25 mCi. Treatment outcome was evaluated six months after therapy using multinomial logistic regression (reference categories: 25 mCi dose and toxic adenoma). RESULTS: The overall cure rate was 80.82%. In patients with Graves' disease, success was achieved in 78.38% of cases, with the majority developing hypothyroidism. The effect of dose was highly significant: an activity of 15 mCi increased the odds of persistent hyperthyroidism 57-fold (p < 0.05), while 10 mCi increased it 1,491-fold (p < 0.01) compared to 25 mCi. Patients with Plummer's disease had 18.36 times higher risk, and those with Graves' disease had 6.09 times higher risk of treatment failure compared to toxic adenoma (p = 0.11 and p = 0.29). CONCLUSION: Doses below 20 mCi, particularly 10 mCi, are associated with unacceptably high failure rates. Multinodular and diffuse forms of hyperthyroidism require higher ablative activities between 20 and 25 mCi.