Outcome of MRI-Guided Single-Dose Iodine-131 for Graves' Hyperthyroidism with Large Goiter.

OBJECTIVE: Surgery is recommended over radioactive iodine-131 therapy (RIT) for Graves’ hyperthyroidism (GH) with large goiter (LG, thyroid volume [TV] ≥ 80 mL). Historically, inaccurate volumetry and insufficient activity yielded low success rates. We aimed to evaluate the efficacy and safety of magnetic resonance imaging (MRI)-guided calculated-activity RIT in LG versus non-LG patients and to identify RIT failure risk factors. METHODS: GH patients receiving calculated-activity RIT (July 2017 to September 2024) were included, excluding those with prior RIT or incomplete data. TV was measured on MRI. Success included overt/subclinical hypothyroidism, euthyroidism, or subclinical hyperthyroidism. Failure included repeat RIT, surgery, or overt hyperthyroidism. Follow-ups ended at second RIT, surgery or March 2025. Risk factors for RIT failure were identified through Firth’s penalized logistic regression and least absolute shrinkage and selection operator. RESULTS: Among 181 patients with GH (mean age, 37.5 ± 13.7 years, 48 men), 33 had LG (mean TV, 146.6 mL). Minor adverse events were observed in LG patients (head-neck swelling [6.1%], exophthalmos progression [3.0%] and new-onset [3.0%]). Success rates were similar (LG 84.8% vs. non-LG 91.9%; P = .36), with universal goiter reduction. Planned activity-to-TV ratio (PA/TV) (odds ratio [OR], 0.68; 95% CI, 0.45–0.94; P = .02), and age (OR, 1.05; 95% CI, 1.01–1.10; P = .01) predicted RIT failure. Planned activity increased with TV in RIT success patients. CONCLUSIONS: With mild adverse events, MRI-guided calculated-activity RIT achieved comparable success in LG and non-LG GH patients. Insufficient PA/TV, but not TV itself, was associated with RIT failure.