Abstract
We treated a 43-year-old woman with Graves' disease (GD), complicated by thymic hyperplasia. Routine chest radiography revealed an abnormal opacity, and computed tomography revealed an anterior mediastinal tumor. At the initial visit to our institution, blood tests revealed hyperthyroidism with positive thyroid-stimulating hormone receptor antibodies, confirming the diagnosis of GD. As the patient opted for long-term antithyroid drug therapy, radioiodine therapy was administered. A computed tomography-guided biopsy revealed thymic hyperplasia. Following radioiodine therapy, the thyroid function normalized, and the tumor significantly shrank. Based on the pathology and clinical course, the patient was diagnosed with thymic hyperplasia secondary to GD, which regressed with conservative management.