Hyponatremia (British: hyponatraemia) is an electrolyte disturbance (disturbance of the salts in the blood) in which the sodium (Natrium in Latin) concentration in the plasma is lower than normal (hypo in Greek; in this case, below 135 mmol/L).
Severe or rapidly progressing hyponatremia can result in swelling of the brain (cerebral edema), and the symptoms of hyponatremia are mainly neurological. Hyponatremia is most often a complication of other medical illnesses in which either fluids rich in sodium are lost (for example because of diarrhea or vomiting), or excess water accumulates in the body at a higher rate than it can be excreted (for example in polydipsia or syndrome of inappropriate antidiuretic hormone, SIADH). There may also be spurious hyponatremia (pseudohyponatremia or factitious hyponatremia) if other substances expand the serum and dilute the sodium (for example, high blood levels of fats in hypertriglyceridemia or high blood sugar in hyperglycemia).