SUN-571 Thymic Hyperplasia: An Unusual Manifestation of Graves’ Disease

Background: The association of Graves’ Disease with thymic hyperplasia is often underrecognized, even though it was first described more than 100 years ago. We hereby present a case of a hyperthyroidism and thymic hyperplasia where surgery could have been avoided. Clinical Case: A 43-year-old female presented with complaints of palpitations, increased sweating, tremors and fatigue for 6 months. Labs showed undetectable TSH with elevated free T4 of 2.3 ng/dl (reference range 0.82-1.77 ng/dl) and free T3 7.7 pg/ml (reference range 2 - 4.4 pg/ml). TSI antibodies were elevated at 617 (Normal <150 %). Due to a previous history of lung nodules, the patient underwent a surveillance CT scan of the chest concurrently. This CT chest showed an anterior mediastinal mass measuring 5 cm×3 cm. The mass was highly suspicious for being thymic in origin. This mass was not noticed on the CT chest done a year ago. PET scan revealed hypermetabolic activity in the thymic and thyroid regions. It was recommended to the patient that this mass likely was thymic hyperplasia in the setting of Graves’ Disease which was amenable to conservative management. However, the patient chose to undergo trans-sternal thymectomy instead of active surveillance. Pathology showed true thymic hyperplasia and no evidence of malignancy. Conclusion: This case illustrates the benign nature of thymic enlargement in Graves’ Disease. The presence of thyrotropin receptors on the thymic epithelial cells which participate in the autoimmune response, leading to hyperplasia in Graves’ disease has been implicated in the pathogenesis of thymic enlargement (1). Pathologically, two distinct features are seen in the thymus in Graves’ disease: lymphoid hyperplasia and true thymic hyperplasia (2). In both these conditions, the thymus gland appears diffusely enlarged. The treatment of thymic enlargement in this condition is the treatment of Graves’ disease, thereby avoiding the need for surgical interventions. References: (1) Murakami, M., Hosoi, Y., Negishi, T., Kamiya, Y., Miyashita, K., Yamada, M., Iriuchijima, T., Yokoo, H., Yoshida, I., Tsushima, Y., Mori, M. Thymic hyperplasia in patients with Graves' disease. Identification of thyrotropin receptors in human thymus. J Clin Invest. 1996 Nov 15;98(10):2228-34. (2) Huang, W., Molitch, M.E. Enlarged Thymus in a Patient with Dyspnea and Weight Loss. JAMA.2015;313(21):2174-2175.