what causes low sodium levels in blood
1 Drinking too much water. Because you lose sodium through sweat, drinking too much water during endurance activities, such as marathons and triathlons, can dilute the sodium content of your blood. Drinking too much water at other times can also cause low sodium.  Dehydration.
Many factors can cause low blood sodium. Sodium levels may get too low if your body is losing too much water and electrolytes. It may also be a symptom of certain medical conditions. Causes of low sodium include: