Hyponatremia is commonly defined as a serum sodium concentration below 135 meq/L but can vary to a small degree in different clinical laboratories [1,2]. The dilutional fall in serum sodium is in most patients associated with a proportional reduction in the serum osmolality (ie, to a level below 275 mosmol/kg), but there are some exceptions. (See 'Hyponatremia with a high or normal serum osmolality' below.)
In virtually all patients, hyponatremia results from the intake (either oral or intravenous) and subsequent retention of water . A water load will, in normal individuals, be rapidly excreted as the dilutional fall in serum osmolality suppresses the release of antidiuretic hormone (ADH, also called vasopressin) (figure 1), thereby allowing excretion of the excess water in a dilute urine. The maximum attainable urine volume in normal individuals on a regular diet is over 10 L/day. This provides an enormous range of protection against the development of hyponatremia since the daily fluid intake in most healthy individuals is less than 2 to 2.5 L/day.
In contrast to the response in normal individuals, patients who develop hyponatremia typically have an impairment in renal water excretion, most often due to an inability to suppress ADH secretion. An uncommon exception occurs in patients with primary polydipsia who can become hyponatremic because they rapidly drink such large quantities of fluid that they overwhelm the excretory capacity of the kidney even though ADH release is appropriately suppressed.
An overview of the causes of hyponatremia will be presented here (table 1). Most of the individual causes of hyponatremia are discussed in detail separately, as are issues related to the diagnosis and treatment of hyponatremia [3,4]. (See "Evaluation of adults with hyponatremia" and "Overview of the treatment of hyponatremia in adults".)
DETERMINANTS OF THE SERUM SODIUM CONCENTRATION
Understanding the factors that determine the serum sodium concentration is required to appreciate the factors that promote the development of hyponatremia and what the composition of intravenous fluids must be to correct the hyponatremia.