FRI564 Thymic Hyperplasia In Graves' Disease: An Under-recognized Association

Disclosure: S. Nachum: None. A. Rivadeneira: None. K.N. Kobaly: None. Association of thymic hyperplasia (TH) with Graves’ disease (GD) is rare and underrecognized. In an era of increased diagnostic imaging, incidental findings of thymic masses in patients with GD may be increasingly identified and awareness of this association may reduce unnecessary interventions. We report three cases of GD with TH at presentation, one of which includes resolution of TH. CASE 1: 56 year-old man with unintentional weight loss, palpitations and eye discomfort was diagnosed with GD (TSH <0.015, FT4 4.47, TRAB 8.18, TSI 8.51). He had an incidental 3x3 cm anterior mediastinal mass on a chest CT. MRI chest suggested TH. Myasthenia gravis antibodies were negative. I-123 uptake and scan showed diffusely increased uptake within the thyroid gland and no increased uptake in the upper chest corresponding to mediastinal mass. Initiation of methimazole normalized thyroid function. CT chest at 9 months showed near resolution of the anterior mediastinal soft tissue density compatible with resolving TH. CASE 2: 23 year-old woman presented with insomnia, palpitations, and neck pain and was found to have GD (TSH <0.01, FT4 2.75, TRAB 9.16, TSI 8.84). CT neck obtained due to pain showed diffuse enlargement of the thyroid and a soft tissue density in the partially imaged superior mediastinum. Methimazole was started. Further evaluation of mass is pending. CASE 3: 35 year-old man presented with atrial fibrillation and was diagnosed with hyperthyroidism (TSH < 0.01, FT4 5.55). Tc-99m pertechnetate scan showed diffuse uptake consistent with GD. Methimazole was started. CT chest obtained due to concern for pulmonary embolism showed a 12.6 x 3.0 cm anterior mediastinal mass. HCG was undetectable. Further evaluation of mass is pending. Discussion: TH is thought to occur due to hormonal excess stimulating the thymic cortex and immunological mechanisms that result in formation of medullary lymphoid follicles.1 The incidence of TH in GD is unknown as diagnostic imaging is not routinely obtained. A study of thymic biopsies in patients undergoing thyroidectomy found 38% of patients with thyrotoxicosis had histological evidence of TH.2 This benign condition improves with treatment of GD, with decreased thymic size after 5-24 months of treatment with antithyroid drugs.3 In the absence of a discrete mass, invasion of other tissues or calcifications, repeat imaging after 6 months with regression of TH by 50% obviates the need for biopsy.1 Biopsy and thymectomy should be deferred when thyrotoxic due to risk of precipitating thyroid storm4 and be reconsidered if thymus fails to regress on repeat imaging. Additional guidance for observation is needed as the differential diagnosis for mediastinal mass includes diseases warranting more urgent diagnosis and intervention. Identification of high-risk patient characteristics or imaging findings may be of clinical utility in this circumstance. Presentation: Friday, June 16, 2023