Effect of 131 Iodine Therapy on the Course of Graves' Ophthalmopathy: A Quantitative Analysis of Extraocular Muscle Volumes Using Orbital Magnetic Resonance Imaging

Abstract

There remains uncertainty as to the effect of radioactive iodine (¹³¹I) therapy on the associated ophthalmopathy (GO). Twenty newly diagnosed patients with Graves' hyperthyroidism treated with¹³¹I (median dose, 15.5 mCi) were followed with ophthalmologic evaluations (OE) and magnetic resonance imaging (MRI) at baseline, 2, and 6 months, and with OE alone at 3 years. For MRI, the superior, inferior, and medial rectus muscle volumes and total muscle volumes (TMV) were measured. Replacement levothyroxine was initiated as low thyroxine (T₄) levels were noted. At baseline, 10 patients(50%) showed evidence of mild GO by OE and/or MRI. There was a significant difference in TMV between the 20 patients with Graves' hyperthyroidism and 10 controls (mean ± standard error [SE]; 2,652± 118 vs. 2,046 ± 96 mm³; P = 0.002) and between the 10 patients with and 10 without GO (3,006 ± 96 vs. 2,298 ± 61 mm³; P = 0.001). TMV correlated with the Hertel score (r = 0.56, P = 0.01). TMV showed no significant change at 2 or 6 months posttreatment. The inferior rectus volume increased slightly at 2 months posttreatment (P= 0.03) but remained stable at 6 months. Furthermore, no significant changes occurred in Hertel scores or in clinical assessments up to 3 years posttreatment and none showed worsening or new development of GO. In conclusion, our results show no significant risk for radioiodine-induced initiation or progression of mild GO.