PSAT340 Thymic Hyperplasia in Graves’ Disease: A Case Report

BACKGROUND: Thymic hyperplasia (TH) can occur in Graves’ disease (GD) due to direct effect of thyroid hormone or thyrotropin receptor antibody stimulation. Recognition of this entity is important as treatment of GD can lead to reduction in TH and can prevent unnecessary intervention. This case highlights thymus enlargement likely due to TH in the setting of GD. CASE: A 44-year-old female with history of left-sided invasive breast cancer status post partial mastectomy and radiation therapy, was found to have an anterior mediastinal mass on surveillance breast MRI. Follow-up chest CT and MRI showed a homogenous anterior mediastinal mass (4.7×2.4×12.6 cm) and thyromegaly. She had a prior history of Hashimoto's thyroiditis diagnosed 20 years, not on treatment, as well as active tobacco use (0.5 PPD for 10 years). She had symptoms of weight loss, palpitations, heat intolerance and tremors. Physical examination revealed tachycardia, flushing, lid lag, diffusely enlarged thyroid and fine tremors. Laboratory investigation showed TSH < 0.01 uIU/mL (0.35-4.94 uIU/mL), free T4 2.84 ng/dL (0.5-1.39 ng/dL), total T3 479 ng/dL (48-178 ng/dL), positive TPO, TSI and thyroglobulin antibodies confirming GD. She was initially treated with methimazole and propranolol with plans for thymectomy. Treatment was complicated by methimazole induced agranulocytosis with absolute neutrophil count of 114 cells/uL (1500-7800 cells/uL) requiring hospitalization. Methimazole was discontinued, and she was treated with antibiotics, propranolol, prednisone, cholestyramine and potassium iodide before undergoing total thyroidectomy. Pathology was consistent with GD with an incidental 2 mm papillary thyroid microcarcinoma. Post-operatively she was started on levothyroxine. Follow up CT chest two weeks after surgery showed a decrease in size of the mediastinal mass supporting a diagnosis of TH secondary to GD. She is planned for repeat imaging in several months to determine if thymectomy is indicated. DISCUSSION: TH can occur in GD secondary to direct trophic effect from excess thyroid hormones or thyrotropin receptor antibody stimulation. Prevalence is unknown; however, studies have shown that approximately one-third of patients with GD have histological evidence of TH. In most documented cases, thymic enlargement is minimal; a 2014 study found 107 documented cases of GD related thymus enlargement, of which only 4 were malignant. Since anterior mediastinal masses can have a high malignancy potential, it is warranted to thoroughly evaluate the radiographic characteristics for malignancy including heterogeneous enhancement, invasion to surrounding structures, and presence of calcification. However, in the presence of untreated hyperthyroidism and absence of suspicious radiographic features, tissue biopsy or thymectomy should be avoided due to high risk of complications such as thyroid storm. TH in GD is often reversible with treatment of GD. Follow up chest imaging to look for decrease in size of thymus is recommended to reevaluate future need for biopsy or thymectomy. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.