Hypernatremia

Hypernatremia is an electrolyte disorder characterized by a serum sodium (Na⁺) concentration >145 mEq/L due to loss of water (ie, dehydration) or gain of sodium.  Although the absolute amount of sodium in the body may be normal, low, or high, hypernatremia always indicates a relative deficit of total body free water (TBFW) relative to total body sodium (TBNa⁺).  This is in contrast to hyponatremia (discussed in a separate article), where there is an excess of TBFW relative to TBNa⁺.

Na⁺ is the primary extracellular cation and plays a critical role in maintaining extracellular fluid volume and osmolality.  The body tightly regulates serum Na⁺ concentration through mechanisms involving the following organs:

The hypothalamus contains osmoreceptors that, when stimulated by increased serum osmolality, trigger thirst.  This prompts increased fluid intake to help maintain proper sodium concentration.