Hyponatremia (American English) or hyponatraemia (British English) is an electrolyte disturbance in which the sodium concentration in the serum is lower than normal. Sodium is the dominant extracellular cation and cannot freely cross the cell membrane. Its homeostasis is vital to the normal physiologic function of cells. Normal serum sodium levels are between 135 and 145 mEq/L. Hyponatremia is defined as a serum level of less than 135 mEq/L and is considered severe when the serum level is below 125 mEq/L.

In the vast majority of cases, hyponatremia occurs as a result of excess body water diluting the serum sodium.

Hyponatremia is most often a complication of other medical illnesses in which excess water accumulates in the body at a higher rate than can be excreted (for example in congestive heart failure, syndrome of inappropriate antidiuretic hormone, SIADH, or polydipsia). Sometimes it may be a result of overhydration.

Lack of sodium is virtually never the cause of hyponatremia, although it can promote hyponatremia indirectly. In particular, sodium loss can lead to a state of volume depletion, with volume depletion serving as signal for the release of ADH (anti-diuretic hormone). As a result of ADH-stimulated water retention, blood sodium becomes diluted and hyponatremia results.