The syndrome of inappropriate antidiuretic hormone secretion has only been reported in a few patients with large cell neuroendocrine carcinoma (LCNEC); however, it has never been reported in a patient with LCNEC of the lung, whose serum sodium levels were normalized after surgical resection of the mass. A 63-year-old male presented with a two-day history of dizziness and recent memory loss. On admission, his serum sodium level was 113 mEq/L with a serum osmolality of 236 mosm/kg, a urine osmolality of 441 mosm/kg, and a urine sodium level of 65 mEq/L. His chest computed tomography revealed a 2.7×2.3 cm-sized mass in the left lower lobe. After surgical removal of the mass, his serum sodium concentrations were normalized, and histopathology of the mass revealed LCNEC.
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