Hypovolemic Hyponatremia.

The etiology of hyponatremia is often multifactorial. The most common causes include hypovolemia from gastrointestinal (GI) or other fluid losses, thiazide diuretics, and SIAD [1]. In this chapter, we will discuss hypovolemic hyponatremia, as well as the clinical parameters that help distinguish between hypovolemic and euvolemic states. These include not only the urine [Na+] concentration but also the fractional uric acid excretion, a parameter that can be employed even when diuretics have been prescribed [2,3,4,5,6,7]. Among the common causes of hypovolemic hyponatremia are GI fluid loss, a range of endocrinopathies [7], and thiazide-induced hyponatremia, which is best considered as a distinct entity, in particular because recent data suggest that it has a genetic predisposition. Also, the discontinuation of thiazide is a key step in treatment [2,7]. The management of hypovolemic hyponatremia starts with confirming its presence and determining the underlying cause. Correction focuses on the appropriate use of isotonic fluid to effect volume repletion while avoiding an overly rapid rise in serum [Na+] concentration.