Low blood sodium, called hyponatremia, is a level of sodium in the blood that is lower than normal, states MedicineNet. It occurs when the body loses fluid and sodium, an element the body needs to function properly, or when the sodium amount is normal, but there is excess body fluid.
The normal blood sodium level is 135 to 145 milliequivalents per liter, notes MedicineNet. A person who consumes excess water after a strenuous workout without replacing lost sodium can develop hyponatremia, also called water intoxication. Patients display symptoms such as headache, vomiting, muscle spasms and, in severe cases, coma or death. Treatment includes replacing electrolytes and administering intravenous fluids.