Thymic Involution After Radioiodine Therapy for Graves Disease: Relationships With Serum Thyroid Hormones and TRAb

CONTEXT: The mechanisms of thymic hyperplasia in Graves disease and its involution after radioiodine (I-131) therapy remain unknown. OBJECTIVE: To examine whether computed tomography (CT) findings of the thymus in patients with Graves disease change before and 6 months after I-131 therapy and to elucidate factors that affect these changes. DESIGN, SETTING: A retrospective, single-center study was conducted. Thymic and thyroid volumes and thymic density were measured on CT. The associations of thymic volume or density with the following factors before I-131 therapy were examined: age; serum triiodothyronine (T3), thyroxine (T4), and thyroid-stimulating hormone receptor antibody (TRAb) levels; and thyroid volume. The changes in thymic volume and density and TRAb levels before and after I-131 therapy, and the correlations of thymic volume with T3 and T4 decline rates and TRAb changing rate and age were examined. PATIENTS: We studied 40 consecutive patients with Graves disease who underwent neck and chest CT before and 6 months after I-131 therapy. INTERVENTION: Observational study. RESULTS: A significant negative correlation was observed only between thymic density and age before I-131 therapy. Thymic volume and density decreased and TRAb levels increased significantly after I-131 therapy. The thymic volume decline rate significantly positively correlated with serum T3 and thyroid volume decline rates. No significant correlation was found between thymic volume decline and TRAb changing rates. CONCLUSIONS: Significant thymic involution occurs after I-131 therapy in patients with Graves disease. Serum T3, but not TRAb, may be related to thymic hyperplasia and involution following I-131 therapy.