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The clinical manifestations of lung cancer are diverse. Presentation initially as the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is relatively uncommon. This article reports the diagnosis and treatment process of a 76-year-old male patient with hyponatremia, ultimately diagnosed with SIADH attributed to small cell lung cancer (SCLC). The patient presented with fatigue and significant hyponatremia. Comprehensive investigation excluded adrenal, thyroid, and other functional abnormalities. Combined with features including high urinary sodium, euvolemic status, and normal renal function, the findings were consistent with a diagnosis of SIADH. Subsequent imaging and pathological examinations revealed the etiology to be SCLC. Management included fluid restriction, sodium supplementation, and the use of tolvaptan. Subsequent chemotherapy and radiotherapy for the SCLC led to the gradual normalization of serum sodium. This article reviews the literature in conjunction with this case, summarizing the diagnostic approach to hyponatremia, as well as the diagnostic workup, pathophysiological mechanisms, and treatment strategies for SIADH, aiming to enhance the early recognition and comprehensive management of this condition.