There are two causes of hyponatremia in small cell lung cancer. 1, various causes of cancer, such as hyponatremia caused by consumption, nausea, vomiting, and malnutrition of malignant tumors; 2, detection of elevated and increased secretion of antidiuretic hormone and cardiac natriuretic peptide. The non-metastatic extrathoracic manifestations of common small cell lung cancer, which can appear before and after the discovery of lung cancer, also called paraneoplastic syndrome, cause renal distal tubules and collecting ducts to prompt water reabsorption and inhibit sodium absorption, manifesting as hyponatremia and abnormal urinary sodium excretion. When hyponatremia is severe enough, clinical manifestations such as altered mental status, disturbance of consciousness, anxiety and coma may occur. Most patients have symptoms that resolve within 1-4 weeks after initial chemotherapy. About 10% of patients present with such symptoms, which may appear as the first symptoms, and some others may have no clinical symptoms.