The main causes of blood sodium below 130mmol/L are too little sodium intake, too much sodium excretion, and too much water in the body. 1. Excessive sodium intake: generally seen in patients with low-sodium diet and no-sodium diet. 2. Excessive sodium excretion: only replenishing water after a large amount of sweating, adrenocortical insufficiency, renal disease, long-term use of diuretics, vomiting, diarrhea, gastrointestinal decompression, etc., which will lead to excessive sodium excretion and cause hyponatremia. 3. Excessive water in the body: excessive water intake, the kidneys may not be able to discharge the body in time. A large amount of pleural fluid and ascites, fluid will accumulate in the third interstitial space, may also lead to the accumulation of sodium ions in these areas, causing hyponatremia. Hyponatremia should be seen promptly.