Co-occurrence of Ectopic Adrenocorticotropic Hormone (ACTH) Secretion and Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) in Small Cell Lung Cancer: A Case Report.

Small cell lung cancer (SCLC) is a neuroendocrine malignancy often associated with paraneoplastic syndromes, such as syndrome of inappropriate antidiuretic hormone secretion (SIADH) and ectopic Cushing's syndrome (ECS). While each is individually recognized, their concurrent presentation is exceptionally rare and poses diagnostic and management challenges. A 49-year-old male with a complex medical history presented with back pain, profound hyponatremia (sodium 119 mmol/L), and hypokalemia (potassium 2.5 mmol/L). Imaging revealed a large right hilar mass with mediastinal lymphadenopathy and hepatic lesions. Biopsy confirmed high-grade neuroendocrine carcinoma consistent with SCLC. Persistent electrolyte abnormalities and treatment-resistant hypertension prompted an endocrinologic workup, revealing elevated cortisol (> 64 μg/dL) and adrenocorticotropic hormone (ACTH) (377 pg/mL), consistent with ectopic ACTH production. Concurrent SIADH was diagnosed based on low serum osmolality, high urine osmolality, and urine sodium > 30 mmol/L. The patient was treated with carboplatin and etoposide as per oncology recommendations. Management of SIADH and ECS was challenging due to the complex interplay of fluid retention, cortisol excess, and risks associated with vasopressin antagonists and cortisol-lowering agents. Chemotherapy was prioritized to address the underlying tumor and paraneoplastic processes. This rare case of concurrent SIADH and ECS in SCLC underscores the importance of early recognition and multidisciplinary management. Awareness of overlapping paraneoplastic syndromes is critical for timely diagnosis and effective treatment, which can prevent life-threatening complications and improve outcomes.