Hyponatremia (American English) or hyponatraemia (British English) is low sodium concentration in the blood. Normal serum sodium levels are between approximately 135 and 145 mEq/liter (135 - 145 mmol/L). Hyponatremia is generally defined as a serum sodium level of less than 135 mEq/L and is considered severe when the serum sodium level is below 125 mEq/L.