Lung tumors causing sodium deficiency can be benign or malignant. To identify the benign or malignant nature of lung tumors a pathocentesis should be done. Both benign and malignant lung tumors that secrete antidiuretic hormone can lead to increased renal sodium excretion and cause hyponatremia. Some patients may have hyponatremia due to decreased appetite, impaired digestion and absorption, or diarrhea caused by lung tumors, resulting in malnutrition of the organism. The most common lung tumor that can secrete antidiuretic hormone is small cell lung cancer, and patients with hyponatremia should be alert to the occurrence of malignant tumors. Patients with hyponatremia should be treated with water restriction, sodium supplementation and appropriate diuretic therapy. Pathological puncture biopsy is the gold standard for diagnosing the benign and malignant of lung tumors. When patients with lung tumors show hyponatremia, they should go to hospital in time, improve the examination, and make clear the nature of the tumor before doing further treatment.