SAT-603 Graves' Disease and an Anterior Mediastinal Mass

Introduction The association between Graves’ disease (GD) and an anterior mediastinal mass (AMM) due to thymic hyperplasia (TH) has been reported. The exact incidence of TH in GD is unknown as chest imaging is not routinely performed. TH in GD typically has a benign course and majority of cases resolved spontaneously with the treatment of GD. However, lack of familiarity of such association may lead to unnecessary surgical intervention. Clinical case A 33-year-old African American lady with Sjogren’s syndrome was presented with progressive shortness of breath and chest discomfort. Exam showed sinus tachycardia, with bilateral exophthalmos and a diffusely enlarged goiter. CTA chest with no pulmonary embolism, however, there was a 11x1.9 cm homogenous mass at anterior mediastinum. She has Graves’ disease, diagnosed 5-year ago with history of non-adherence to methimazole. TFT showed TSH of <0.1 uIU/ml (ref 0.34-5.6), FT4 of 5.38 ng/dl (ref 0.6-1.6), FT3 of >20 pg/ml (ref 2.3-4.2). TSI was elevated to 365% (ref <140). She had thoracoscopic partial resection of anterior mediastinal mass at outside hospital and reported as thymus hyperplasia. &nbsp; Discussion Differential diagnosis of AMM include retrosternal goiter, thymoma, thymic carcinoma, lymphoma and germ cell tumor. TH is the most common etiology of AMM in GD. The risk of thymic malignancy in GD appeared to be low at 3.7% in one study. In a study by Murakami et al, thyrotropin receptors were present in thymic tissue and GD patients had larger thymic size and higher thymic density than age-matched control subjects. TH in GD can be from lymphoid hyperplasia (TLH) due to increased number of lymphoid follicles with germinal centers or true thymic hyperplasia (TTH) from actual enlargement of the thymus. Thymic enlargement is more common in TTH. The mechanism of TH in GD is appeared to be thyroid hormone dependent which affect more on cortex causing TTH and immunologic process which affect more on medullary lymphoid follicles causing TLH. The degree of TH is correlated with the severity of thyrotoxicosis and the level of TSH receptor antibodies (TRAb).&nbsp; Treatment with anti-thyroid causes regression of TH by reducing levels of thyroid hormone and TRAb. In a literature review by Haider et al, 31 out of 47 GD-associated TH patients had an average of 62% reduction in thymic volume with the treatment of GD. The time to regression of TH under medical treatment varies considerably from 2 to 24 months, however, majority of cases showed >50% volume reduction at six months of therapy.&nbsp; &nbsp; Clinical lesson As TH in GD is a benign reversible autoimmune process, AMM in GD can be followed with repeat imaging. Thymic biopsy may be considered only if the size of the mass does not decrease with euthyroid state or patients with concerning radiological features of AMM such as cystic or necrotic changes, invasion to surrounding structure, or calcification. &nbsp;