http://www.mayoclinic.org/diseases-conditions/hyponatremia/basics/causes/CON-20031445
Certain medications.
Some medications, such as some water pills (diuretics), antidepressants and pain medications, can cause you to urinate or perspire more than normal.
Heart, kidney and liver problems.
Congestive heart failure and certain diseases affecting the kidneys or liver can cause fluids to accumulate in your body, which dilutes the sodium in your body, lowering the overall level.
Syndrome of inappropriate anti-diuretic hormone (SIADH).
In this condition, high levels of the anti-diuretic hormone (ADH) are produced, causing your body to retain water instead of excreting it normally in your urine.
Chronic, severe vomiting or diarrhea.
This causes your body to lose fluids and
electrolytes, such as sodium.
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.
Taking in too little fluid can also be a problem. If you get dehydrated, your body loses fluids and electrolytes.
Hormonal changes.
Adrenal gland insufficiency (Addison's disease) affects your adrenal glands' ability to produce hormones that help maintain your body's balance of sodium, potassium and water. Low levels of thyroid hormone also can cause a low blood-sodium level.