Sodium is the principal extracellular cation regulating fluid balance, blood pressure, nerve conduction, and muscle contraction. The kidneys maintain serum sodium within a very narrow range through the RAAS, ADH, and ANP systems.

Sodium abnormalities primarily reflect water balance disturbances. Hyponatraemia (low sodium) is the most common electrolyte abnormality in hospitalised patients and can cause neurological symptoms ranging from nausea to seizures and coma. Hypernatraemia (high sodium) usually reflects dehydration or insufficient water intake.