Hyponatremia is a decrease in plasma sodium (Na) levels below 135 mmol/L (mEq/L). This condition has a wide range of causes - from excessive water intake with a salt-free diet, uncontrolled use of drugs to severe kidney disease and malignant tumors. Clinical symptoms are represented mainly by neuropsychiatric disorders. The diagnosis is made on the basis of determining the level of sodium and the osmolarity of blood serum, urine. Treatment should include control of the cause, administration of isotonic or hypertonic saline solutions, and maintenance of euvolemia.