Hyponatremia

Hyponatremia (American English) or hyponatraemia (British English) is an electrolyte disturbance in which the sodium concentration in the serum is lower than normal. (Hypo = low; natraemia = sodium in blood) Sodium is the dominant extracellular cation and cannot freely cross the cell membrane. Its homeostasis is vital to the normal physiologic function of cells. Normal serum sodium levels are between 135 and 145 mEq/L. Hyponatremia is defined as a serum level of less than 135 mEq/L and is considered severe when the serum level is below 125 mEq/L.