Graves disease is an autoimmune disorder characterized by thyroid enlargement and hyperthyroidism. Thyrotropin receptor (TSHR) autoantibodies bind the TSHR on the membrane of thyroid follicular cells and stimulate cell proliferation and thyroid-hormone synthesis. TSHR has also been identified in extrathyroidal organs, including the human thymus. Thus far, radiologically detectable thymic enlargement has only been reported in adults with Graves disease. We present here the case of a child with Graves disease and significant thymic hyperplasia. L. K. was a 15-year-old girl evaluated for cough and dyspnea on exertion. A chest radiograph was obtained, and it revealed a widened superior mediastinum. A computed-tomography scan of her chest identified a mass in the anterior mediastinum without associated lymphadenopathy. Because of these radiologic findings and her weight loss, she was referred to the oncology service; a biopsy of the mediastinal mass was obtained and revealed thymic reactive hyperplasia. Because of persistent tachycardia, thyroid studies were obtained, and the diagnosis of Graves disease was established. The child's physical examination revealed a minimally enlarged thyroid gland and no exophthalmos. One month after medical treatment was initiated, a repeat computed-tomography scan of her chest identified significantly reduced thymic size. To our knowledge, this is the first pediatric patient reported with Graves disease and significant thymic enlargement. Considering the diagnosis of Graves disease for a child with an anterior mediastinal mass and without the typical physical findings of autoimmune hyperthyroidism (goiter, exophthalmos) may prevent unnecessary diagnostic studies and their associated financial and emotional costs.

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