MON-317 - Reversible Thymic Hyperplasia in Graves' Disease

Introduction
Graves’ disease is an autoimmune cause of hyperthyroidism marked by diffuse goiter and thyroid autoantibodies. Thymic hyperplasia is a rare but recognized association and may mimic anterior mediastinal malignancies, creating a diagnostic challenge.

Case presentation
A 35-year-old woman presented with two days of progressive palpitations, fatigue, and fever. She was tachycardic and hypertensive at presentation. Laboratory evaluation showed suppressed thyroid-stimulating hormone (TSH) < 0.01mIU/L (0.55-4.78 mIU/L), elevated free T4 25.87 pmol/L (11.46-22.65 pmol/L), and free T3 11.24 pmol/L (3.85 - 6.01 pmol/L). Antibody testing showed elevated thyroid-stimulating immunoglobulins (TSI) 356% ( < 140%) and thyrotropin receptor antibodies (TRAb) 18.89 IU/L (≤2.0IU/L), with negative thyroid peroxidase (TPO) antibodies. Chest CT revealed an incidental 2.4 × 1.9 cm anterior mediastinal mass concerning for thymic hyperplasia versus thymoma or lymphoma. Tumor markers (AFP, β-hCG) and myasthenia gravis testing were negative. She was started on methimazole but developed urticaria and was transitioned to propylthiouracil, followed by referral for thyroidectomy. Oncology and cardiothoracic surgery collaboratively recommended conservative management with interval imaging. Total thyroidectomy revealed a benign adenomatoid goiter. Serial chest CT at 3, 12, and 24 months post-surgery showed progressive regression of the thymic lesion with near-complete resolution by 24 months.

Conclusion
Thymic hyperplasia in Graves’ disease is thought to result from a combination of autoimmune stimulation and thyroid hormone–mediated thymic proliferation. Reported cases consistently demonstrate regression of thymic enlargement following restoration of euthyroidism, supporting a reversible, hormone-driven process. Thymic enlargement is frequently misinterpreted as malignancy, leading to avoidable invasive testing. This case highlights Graves’-associated thymic hyperplasia as an important diagnostic consideration for physicians evaluating anterior mediastinal masses.

Keywords: Graves’ disease, Thymic hyperplasia, Anterior mediastinal mass

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*