Thyroid volume is the key predictor of hyperthyroidism remission after radioactive iodine therapy in pediatric patients.

Graves' disease (GD) is the leading cause of hyperthyroidism in pediatric patients. Radioactive iodine therapy (RAIT) is widely used to treat GD. However, it is still unclear exactly what determines the efficacy of RAIT in childhood and adolescence. The objective of our study was to reveal the most significant predictors of the efficacy of RAIT in pediatric GD patients. A single-center prospective observational exploratory study enrolled 144 pediatric patients (124 females and 20 males) between 8 and 18 years of age who underwent dosimetry-guided RAIT for GD for the first time. The estimated parameters included sex, age, thyroid volume, thyroid stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4), thyroid-stimulating hormone receptor antibodies (TRABs) at baseline and 12 months after RAIT, 10- to 20-min 99m Tc thyroid uptake (%), maximum thyroid 131 I uptake (%), specific 131 I uptake (MBq/g), and therapeutic activity of 131 I (MBq), which was limited to 1100 MBq. The Fisher's exact test, Mann-Whitney U-test, Wilcoxon signed-rank test, ROC analysis, and the Youden index were used for statistical analysis. Twelve months after RAIT, 119 patients (83%) successfully achieved remission, 6 patients (4%) had euthyroidism, and hyperthyroidism persisted in 19 patients (13%). Thyroid volume decreased from 17.6 [14.6; 24.1] to 9.3 [7.6; 13.3] mL 12 months after the treatment (p < 0.001). The main predictor that showed a statistically significant difference between the groups of patients who achieved and did not achieve remission of GD hyperthyroidism after RAIT was the initial thyroid volume. Using the Youden index, the optimal cut-off point for the initial thyroid volume at 45.4 mL was determined.     Conclusion: The dosimetry-guided RAIT in pediatric GD patients was 83% effective at 12 months after the treatment, and the initial thyroid volume of less than 45.4 mL was the most important predictor of RAIT success. Other predictors identified in our work included FT4 levels, TRABs levels, 99m Tc-pertechnetate uptake, and specific 131 I uptake. What is Known: •Radioiodine therapy is a common, effective, and safe treatment for pediatric patients with Graves' disease. What is New: •The initial thyroid volume in pediatric GD patients is an important predictor of achieving hypothyroidism following radioiodine therapy. If the thyroid volume is less than 45.4 ml, radioiodine therapy limited to 1100 MBq will be effective definitive treatment.