Practice Essentials. Hyponatremia is an important and common electrolyte abnormality that can be seen in isolation or, as most often is the case, as a complication of other medical illnesses (eg, heart failure, liver failure, renal failure, pneumonia). The normal serum sodium level is 135-145 mEq/L. Hyponatremia is defined as a serum sodium level of less than 135 mEq/L. Joint European guidelines classify hyponatremia in adults according to serum sodium concentration, as follows: Mild: 130-134 mmol/L.