If blood sodium is persistently low and not easily corrected by supplementation, it is recommended to check urine sodium. If blood sodium is low and urine sodium is low consider that it is due to gastrointestinal fluid loss and intravenous supplementation is sufficient. If urine sodium is high it should be caused by two possibilities, one is hypoadrenocorticism and the other is inappropriate secretion of antidiuretic hormone syndrome. If there is hypotension, hypoglycemia, hyperkalemia, low blood cortisol urine consider the former, intravenous supplementation of hydrocortisone is needed; if there is no such manifestations tend to the latter, water restriction is conducive to normalization of blood sodium.