Hyponatremia is a potentially serious condition in which the level of sodium in the blood is too low. An imbalance is created when there is too little sodium for the amount of water in the body. As a result, water moves into the body’s cells causing them to swell.

There are different types of hyponatremia, each resulting in low sodium in the body:

  • Euvolemic hyponatremia—water level increases, but sodium level stays the same
  • Hypervolemic hyponatremia—water and sodium levels increase, but the water gain is greater
  • Hypovolemic hyponatremia—water and sodium levels decrease, but the sodium loss is greater

Hyponatremia may be caused by:

  • Syndrome of inappropriate antidiuretic hormone secretion (SIADH)—Antidiuretic hormone signals the kidneys to absorb more water, reducing urine output. In SIADH, the mechanism that stops antidiuretic hormone from collecting water is impaired. This impairment results in excess water in the body.
  • Sweating—In people with cystic fibrosis, excess sodium is excreted through sweat. It may also occur in people with severe burns when electrolytes and fluids are not replaced.
  • Some diuretics—Increase sodium is lost in the urine.

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