In some lung cancer patients, clinical weakness, nausea and vomiting, headache and drowsiness, painful muscle spasms, neuropsychiatric symptoms appear, blood is taken to measure electrolytes and hyponatremia is found, then, which type of lung cancer is most likely to be combined with hyponatremia? In clinical practice, the most common type of lung cancer causing hyponatremia is small cell lung cancer. The main mechanism is that small cell lung cancer can synthesize, secrete and autonomously release antidiuretic hormone, resulting in a significant increase of antidiuretic hormone in the patient’s serum and lung cancer tissue, leading to impaired water excretion, hemodilution, decrease of serum sodium concentration and osmolality, and hyponatremia. Small cell lung cancer combined with hyponatremia is not effective only by simple electrolyte supplementation, and only treatment of the primary disease is the best treatment. If the treatment is effective, hyponatremia will improve, and if it recurs, hyponatremia can occur again.