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| === Pseudohyponatremia ===
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| Certain conditions that interfere with laboratory tests of serum sodium concentration (such as extraordinarily high blood levels of [[lipid]] or [[protein]]) may lead to an erroneously low ''measurement'' of sodium. This is called pseudohyponatremia, and can occur when laboratories use the flame-photometric and indirect (but not direct) ion-selective electrode assays.<ref>Weisberg LS. (1989) Pseudohyponatremia: a reappraisal. Am J Med, 86(3):315-8. PMID 2645773 </ref><ref>Nguyen MK et al. (2007) A new method for determining plasma water content: application in pseudohyponatremia. Am J Phys - Renal, 292(5):F1652-6. PMID 17299138</ref> This is distinct from a true dilutional hyponatremia that can be caused by an osmotic shift of water from cells to the bloodstream after large infusions on [[mannitol]] or [[IVIG|intravenous immunoglobulin]].
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| === Hypoosmolar hyponatremia ===
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| When the plasma osmolarity is low, the extracellular fluid volume status may be in one of three states:
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| *'''Low volume'''. Loss of water is accompanied by loss of sodium.
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| **Excessive [[sweat]]ing
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| **[[Burn (injury)|Burns]]
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| **[[Vomit]]ing
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| **[[Diarrhea]]
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| **[[Urine|Urinary]] loss
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| ***[[Diuretic]] drugs (especially [[thiazide]]s)
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| ***[[Addison's disease]]
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| ***[[Cerebral salt-wasting syndrome]]
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| ***Other salt-wasting [[kidney]] diseases
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| Treat underlying cause and give IV isotonic saline. It is important to note that sudden restoration of blood volume to normal will turn off the stimulus for continued ADH secretion. Hence, a prompt water diuresis will occur. This can cause a sudden and dramatic increase the serum sodium concentration and place the patient at risk for so-called "[[central pontine myelinolysis]]" (CPM). That disorder is characterized by major neurologic damage, often of a permanent nature.
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| Because of the risk of CPM, patients with low volume hyponatremia may eventually require water infusion as well as volume replacement. Doing so lessens the chance of a too rapid increase of the serum sodium level as blood volume rises and ADH levels fall.
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| *'''Normal volume'''.
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| **[[SIADH]] (syndrome of inappropriate [[antidiuretic hormone]])
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| **Some cases of psychogenic [[polydipsia]]
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| The cornerstone of therapy for SIADH is reduction of water intake. If hyponatremia persists, then [[demeclocycline]] (an antibiotic with the side effect of inhibiting ADH) can be used. SIADH can also be treated with specific antagonists of the [[antidiuretic hormone|ADH]] receptors, such as [[conivaptan]] or [[tolvaptan]].
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| *'''High volume'''. There is retention of water.
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| **[[Congestive heart failure]]
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| **[[Hypothyroidism]] and [[hypocortisolism]]
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| **[[Liver]] [[cirrhosis]]
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| **[[Nephrotic syndrome]]
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| **[[Psychogenic polydipsia]]
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| Placing the patient on water restriction can also help in these cases.
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| Severe hyponatremia may result from a few hours of heavy exercise in high temperature conditions, such as hiking in desert areas, or from endurance athletic events when electrolytes are not supplied. (Such an incident notably happened to long-distance athlete Craig Barrett in 1998).
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| ==Diagnosis== | | ==Diagnosis== |