Hyponatremia is not a specific disease, but a laboratory determination associated with a variety of diseases, in which the level of sodium ion circulating in the blood is below normal; the normal range is 136-146 mEq/L. In general, the cause is a disturbance in the mechanisms of water excretion. Total body sodium may be normal, but the ratio between sodium in the intracellular and extracellular components may be inappropriate. 
The initial diagnosis of hyponatremia, as opposed to the underlying disorder, normally comes from a serum electrolyte panel taken from blood. Continuous monitoring, typically in the critical care environment, can come from an indwelling ion selective electrode.
Experienced physicians have difficulty in determining the underlying causes of hyponatremia.
Immediate treatment includes water restriction, and often, but with care, diuretics, usually furosemide, along with isotonic (0.9%) sodium chloride solution (i.e., saline). Dialysis may be appropriate, especially in patients with renal failure.
If the hyponatremic state is causing severe changes, such as coma or seizure, small amounts of hypertonic (3%) saline may be administered intravenously. Hypertonic saline generally is reserved for intensive care units, in which continuous monitoring of central venous pressure or pulmonary capillary wedge pressure is available. Furosemide is usually given with the saline. 
Tolvaptan, a vasopressin receptor antagonist, can be used for hyponatremia. However, vasopressin receptor antagonist may increase the frequency of rapid sodium correction although osmotic demyelination syndrome may not be increased.
- Chansky ME, Nyce A, Friedman J (2004), Chapter 42, Fluid, Electrolyte, and Acid-Base Emergencies, in Stone CK, Humphries CK, Current Emergency Diagnosis and Treatment (Fifth Edition ed.), Lange Medical Books, McGraw-Hill
- Fenske W, Maier SK, Blechschmidt A, Allolio B, Störk S (2010). "Utility and limitations of the traditional diagnostic approach to hyponatremia: a diagnostic study.". Am J Med 123 (7): 652-7. DOI:10.1016/j.amjmed.2010.01.013. PMID 20609688. Research Blogging.
- Chansky, Nyce & Friedman, p. 894
- (2009) Tolvaptan (Samsca) for Hyponatremia The Medical Letter
- Rozen-Zvi B, Yahav D, Gheorghiade M, Korzets A, Leibovici L, Gafter U (2010). "Vasopressin receptor antagonists for the treatment of hyponatremia: systematic review and meta-analysis.". Am J Kidney Dis 56 (2): 325-37. DOI:10.1053/j.ajkd.2010.01.013. PMID 20538391. Research Blogging.