Hyponatremia reflects an excess of total body water (TBW) relative to total body sodium content. Because total body sodium content is reflected by ECF volume status, hyponatremia must be considered along with status of the ECF volume: hypovolemia, euvolemia, and hypervolemia (see Table: Principal Causes of Hyponatremia). Note that the ECF volume is not the same as effective plasma volume. For example, decreased effective plasma volume may occur with decreased ECF volume (as with diuretic use or hemorrhagic shock), but it may also occur with an increased ECF volume (eg, in heart failure, hypoalbuminemia, or capillary leak syndrome).
Sometimes, a low serum sodium measurement is caused by an excess of certain substances (eg, glucose, lipid) in the blood (translocational hyponatremia, pseudohyponatremia) rather than by a water-sodium imbalance.