Missed Diagnosis of Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) as a Paraneoplastic Syndrome in Small Cell Lung Cancer: A Case Report.

Small cell lung cancer (SCLC) is an aggressive malignancy frequently associated with paraneoplastic syndromes, including the syndrome of inappropriate antidiuretic hormone secretion (SIADH). These syndromes usually happen prior to tumor detection and can lead to delayed diagnosis, especially when initial imaging is inconclusive. Here we report the case of a 56-year-old female with a history of chronic tobacco and alcohol use who presented to the emergency department multiple times over two months with symptoms of fatigue, dizziness, and hyponatremia. After several hospital encounters and repeated chest X-rays, an eventual chest CT scan revealed a right hilar mass. The patient was subsequently diagnosed via endobronchial biopsy and started on chemotherapy. This case highlights the diagnostic challenge of paraneoplastic SIADH in SCLC with imaging, especially in patients with confounding risk factors for hyponatremia. Initial negative imaging delayed diagnosis, underscoring the limitations of chest X-rays and the need for high clinical suspicion in high-risk populations. Early CT imaging may expedite diagnosis, reduce delays in treatment, and improve clinical outcomes.