The term dysnatremia applies when an aberration in plasma sodium concentration is present. Changes in plasma sodium concentration can result in fluid shifts between the intra- and extracellular compartments of the body. In the healthy state, the body’s osmoregulatory system maintains the plasma sodium concentration between 135 and 145 mEq/L. Failure of this system begets an imbalance of free water intake and excretion. When free water intake exceeds excretion, the plasma sodium concentration decreases below 135 mEq/L, a condition known as hyponatremia. In contrast, hypernatremia, defined as a serum sodium concentration >145 mEq/L, occurs when electrolyte-free water excretion exceeds intake. Less commonly, pure loss or addition of sodium without a primary disturbance in water balance may also lead to hypo- or hypernatremia.