Malignant Thymoma Associated with Myasthenia Gravis, Graves' Disease, and SIADH

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Patients with thymoma are likely to present with associated autoimmunologic disorders. The occurrence of syndrome of inappropriate antidiuretic hormone (SIADH) attributable to thymoma is extremely rare. We herein present an extremely rare case of a 59-year-old man patient who was discovered to have malignant thymoma associated with myasthenia gravis, Graves' disease, and SIADH. He was admitted for evaluation and treatment of hyponatremia (Na 125 mEq/l). SIADH was diagnosed, and thymoma was identified as its cause. The patient was also found to have both Graves' disease and myasthenia gravis. The hyponatremia was normalized with water restriction and 3% saline therapy before thymectomy. The thymic tumor was a Masaoka stage III thymoma that resulted in direct invasion to the wall of the innominate vein, but there was no finding of invasion to other mediastinal organs. Complete thymectomy with innominate vein graft was performed. Microscopic histopathology findings corresponded to those of a mixed-type thymoma and type B2. However, immunohistochemical stain for antidiuretic hormone was negative in the tumor cells. Adjuvant radiation therapy was employed postoperatively, and the patient's postoperative recovery was uneventful. He subsequently reached a euthyroid state. And the reversal to normal sodium and osmolality levels was continued after the tumor removal without any further management for hyponatremia. The observation of this interesting case and a literature review provided us with the opportunity to explore the pathogenesis and clinical aspects of thymoma-related autoimmune and/or endocrine disorders which must be suspected in patients with thymoma.