What is the normal serum sodium range and what causes hyponatremia?

Normal serum sodium is 136 to 145 milliequivalents per liter. Hyponatremia, below 136, is the most common electrolyte imbalance in hospitalized patients. Causes include excessive water intake, syndrome of inappropriate antidiuretic hormone also called SIADH, heart failure, cirrhosis, diuretic use, vomiting, and diarrhea. Signs include confusion, headache, nausea, lethargy, seizures, and in severe cases brain herniation from cerebral edema. Severe hyponatremia below 120 is a medical emergency. Treatment depends on the cause and chronicity.