Hyponatremia is an electrolyte disturbance in which the sodium concentration in the serum

Blood serum

In blood, the serum is the component that is neither a blood cell nor a clotting factor; it is the blood plasma with the fibrinogens removed...

 is lower than normal. In the vast majority of cases, hyponatremia occurs as a result of excess body water diluting the serum sodium and is not due to sodium deficiency. Sodium is the dominant extracellular cation and cannot freely cross the cell membrane. Its homeostasis is vital to the normal physiologic function of cells.