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Etiology and evaluation of hypernatremia in adults

Richard H Sterns, MD
Section Editor
Michael Emmett, MD
Deputy Editor
John P Forman, MD, MSc


The term "tonicity," also called "effective osmolality," refers to the activity of solutes that do not easily cross cell membranes (effective osmoles) and therefore determine the transcellular distribution of water. Because sodium and its anions make up most of the effective osmoles in the extracellular fluid, a high plasma sodium concentration (hypernatremia) indicates hypertonicity and a decrease in cell volume [1]. In most cases, hypernatremia results from water depletion. This develops when water losses are not replaced because water is unavailable, when the urge to drink is impaired, or because patients cannot seek water for themselves. Abnormally large unreplaced water losses (as an example, due to diabetes insipidus) result in a rapid onset of hypernatremia. However, even with large water losses, hypernatremia will not develop if thirst is intact and water is available. (See 'The importance of thirst' below.)

Although hypernatremia is most often due to water loss, it can also be caused by the intake of salt without water or the administration of hypertonic sodium solutions [2]. (See 'Sodium overload' below.)

Hypernatremia due to water depletion is called dehydration. This is different from hypovolemia, in which both salt and water are lost [2]. (See "General principles of disorders of water balance (hyponatremia and hypernatremia) and sodium balance (hypovolemia and edema)".)

The causes and evaluation of hypernatremia will be reviewed in this topic (table 1). The treatment of patients with hypernatremia is discussed separately. (See "Treatment of hypernatremia".)

Prior to discussing the etiology and evaluation of hypernatremia, it is helpful to review the determinants of the plasma sodium concentration.

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Literature review current through: Sep 2017. | This topic last updated: Dec 10, 2015.
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