Hyponatremia, or low serum sodium, results when the serum sodium falls below 135 mEq/L. The serum osmolality also becomes low, falling below 275 mOsm. In normal renal function the body eliminates excess water by decreasing the release of antidiuretic hormone (see Chapter 8). When this homeostatic mechanism malfunctions, excess water is retained and fluid shifts occur. When more fluid than sodium is present in the intravascular compartment, fluid shifts via osmosis to the more concentrated area inside the cell.

Conditions resulting in an excessive sodium loss or excessive water gain lead to hyponatremia.
It may also be caused by inadequate sodium intake, although this is atypical and is more likely to occur in patients who are on salt-restricted diets or taking diuretics, such as the elderly.