Sodium plays a key role in your body. It helps maintain normal blood pressure, supports the work of your nerves and muscles, and regulates your body's fluid balance.

When the sodium level in your blood becomes too low, extra water enters your cells and causes them to swell. Swelling in your brain is especially dangerous because the brain is confined by your skull and unable to expand without causing symptoms.

Types of hyponatremia

A normal sodium level is between 135 and 145 milliequivalents per liter (mEq/L) of sodium. Hyponatremia occurs when the sodium in your blood falls below 135 mEq/L.

The imbalance between sodium and water in your blood may occur in three primary ways:

  • In hypervolemic hyponatremia, your body has too much water. Hypervolemic hyponatremia is commonly the result of kidney failure, heart failure or liver failure.
  • In euvolemic hyponatremia, your water level is too high. This condition is commonly due to chronic health conditions, cancer or certain medications.
  • In hypovolemic hyponatremia, you have too little water and sodium. This may occur, for example, when exercising in the heat without replenishing your fluid electrolytes or with marked blood loss.

Causes of hyponatremia

Many possible conditions and lifestyle factors can lead to hyponatremia, including:

  • Certain medications. Some medications, such as some antidepressants and pain medications, can cause you to urinate or perspire more than normal.
  • Water pills (diuretics) — especially thiazide diuretics. Diuretics work by making your body get rid of more sodium in urine.
  • Cirrhosis. Liver disease can cause fluids to accumulate in your body.
  • Kidney problems. Kidney failure and other kidney disease may make it hard to efficiently remove extra fluids from your body.
  • Congestive heart failure. This condition causes your body to retain fluids.
  • 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.
  • Drinking too much water during exercise (exertional hyponatremia). 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.
  • Hormonal changes due to adrenal gland insufficiency (Addison's disease). Your adrenal glands produce hormones that help maintain your body's balance of sodium, potassium and water.
  • Hormonal changes due to an underactive thyroid (hypothyroidism). Hypothyroidism may result in a low blood-sodium level.
  • Primary polydipsia. In this condition, your thirst increases significantly, causing you to drink too much fluid.
  • The recreational drug Ecstasy. This amphetamine increases the risk of severe and even fatal cases of hyponatremia.
  • Chronic, severe vomiting or diarrhea. This causes your body to lose fluids and electrolytes, such as sodium.
  • Dehydration. In dehydration, your body loses fluids and electrolytes.
  • Diet. A low-sodium, high-water diet can sometimes disturb the proper balance between sodium and fluids in your blood.
Jul. 14, 2011