SAT-357 Massive Thymic Hyperplasia in Graves' Disease

Abstract Disclosure: C.C. Chan: None. S. Ryder: None. D. Luong: None. G. Puri: None. First described in 1914, thymic hyperplasia arising from Graves’ disease is a rare but well-established complication with at least 100 previous reports in the literature. The pathophysiology of thymic hyperplasia in Graves’ disease is not entirely understood. However, thyroid hormone has been shown to exert a trophic effect on thymic epithelial and lymphoid cells. Furthermore, thyrotropin receptor antibodies may also stimulate extra-thyroidal thyrotropin receptor, which have been demonstrated to exist on the thymus. Notably, detection of a discrete mass, invasiveness, and calcification on imaging should prompt biopsy or further imaging as the differential diagnosis of an anterior mediastinal mass includes lymphoma, germ-cell and mesenchymal tumours, thyroid and parathyroid masses, and metastatic malignancy. However, it is well established that thymic hyperplasia regresses with adequate treatment of Graves’ disease, typically with anti-thyroid drugs. Thus, in the absence of concerning imaging findings, the literature supports medical management of an anterior mediastinal mass in Graves’ disease in the first instance. We report the case of a Burmese refugee with newly diagnosed Graves’ disease that had a massive thymic mass measuring 17mm x 21mm x 121mm (43.2cm3) incidentally detected on imaging. As the size of the mass exceeded typical reports in the literature, she was broadly investigated for autoimmune conditions and malignancy. The massive size of the mass was not accompanied by a significant thyrotoxicosis, consistent with previous reports demonstrating no dose-dependent relationship between thymic volume and thyroid hormone or thyrotropin receptor antibody. Computed Tomography-guided biopsy was initially considered given the presence of lymph node Fluorodeoxyglucose-avidity. However, with clear regression of the mass on serial imaging and Chemical-shift MRI demonstrating a signal loss uncharacteristic of malignancy, she was treated conservatively with continuation of antithyroid drugs, and a presumptive diagnosis of thymic hyperplasia was made. Recognition of this association is vital to prevent unnecessary biopsy and surgical intervention. Presentation: Saturday, July 12, 2025