Symptomatic syndrome of inappropriate anti-diuretic hormone as a rare early presentation of primary thymic carcinoma: a case report

Syndrome of inappropriate anti-diuretic hormone (SIADH) can be presented as a paraneoplastic syndrome in primary malignancies involving the lung and brain. However, the development of SIADH in primary thymic carcinoma is poorly documented. We report a case of an elderly, with an initial presentation of symptomatic persistent hyponatremia as a paraneoplastic syndrome of SIADH with an incidental finding of anterior mediastinal mass confirmed on imaging. Further investigations are consistent with the diagnosis of poorly differentiated locally advanced thymic carcinoma with lung infiltration (T3N1Mx). The patient underwent an En-bloc total thymectomy and subsequently completed adjuvant chemotherapy and further follow-up showed a complete resolution of hyponatraemic SIADH. In conclusion, SIADH may be presented as a paraneoplastic syndrome in primary thymic carcinoma and early detection of thymic malignancy is paramount to ensure early diagnosis and prognostication.