Central Diabetes Insipidus as a Rare Presentation of Small Cell Lung Cancer: A Case Report.

Small cell lung carcinoma (SCLC) is an aggressive neuroendocrine malignancy commonly associated with paraneoplastic syndromes, most often the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Central diabetes insipidus (CDI) is a rare manifestation and may result from hypothalamic-pituitary involvement by metastatic disease. A 53-year-old man with a significant smoking history presented with abdominal pain and was found to have a right perihilar lung mass with extensive metastatic disease. Liver biopsy confirmed SCLC. During hospitalization, he developed acute polyuria with worsening hypernatremia, elevated serum osmolality, and inappropriately dilute urine. A marked concentrating response following desmopressin administration supported the diagnosis of CDI, while MRI of the brain demonstrated a sellar-suprasellar mass with laboratory findings concerning for central hypothyroidism, indicating hypothalamic-pituitary axis disruption. Given the rapid onset of symptoms and widespread metastatic disease, pituitary metastasis was considered the most likely etiology. Despite initiation of desmopressin and systemic chemotherapy, the patient experienced progressive clinical decline and ultimately transitioned to comfort care. This case highlights a rare presentation of SCLC with CDI likely due to pituitary metastasis. New-onset polyuria and hypernatremia in patients with advanced malignancy should prompt evaluation for CDI to allow timely diagnosis and management, even in the setting of poor overall prognosis.