ISSN 2398-2969      




  • Definition: serum sodium concentration <140 mEql/L.
    Normal ranges and abnormal values are dependent on equipment used and reference ranges established for that equipment.
  • Sodium major cation of extracellular fluid (ECF).
  • It does not freely move into the intracellular space (ICF) but is dependent on the action of the Na/K ATPase and is therefore the major determinant of plasma osmolality.
  • Water moves freely between the ECF and ICF following an osmotic gradient.
  • Measured sodium concentration does not reflect total body sodium content but indicates the amount of sodium relative to the amount of the ECF water.
  • Hyponatremia usually implies hypo-osmolality.


Predisposing factors

  • None.


  • None.


  • Serum sodium concentration is a reflection of the amount of sodium relative to the volume of total body water.
  • Hyponatremic patients may have decreased, increased or normal total body sodium content. Evaluation of sodium should be undertaken alongside their volume status.
  • Pseudohyponatremia:
    • Artifact when sodium measured by flame photometry in severely lipemic or hyperproteinemic samples.
  • Hyponatremia may occur with increased plasma osmolarlty (unusual). This happens as a result of the addition of an effective osmole to the ECF and shift of water from the ICF to the ECF.
  • Hyponatremia with decreased plasma osmolality (more common). It may be accompanied by normal, decreased or increased plasma volume.
    • Normal plasma volume.
    • Psychogenic polydipsia.
    • Syndrome of inappropriate ADH secretion (SIADH).
    • Drugs with antidiuretic effects (ie barbiturates, narcotics and vincristine, tricyclic antidepressants, beta adrenergics, cholinergics, NSAIDs, oxytocin, phenothiazine, cyclophosphamide, serotonin reuptake inhibitors).
    • Myxedema coma (hypothyroidism Hypothyroidism ).
    • Administration of hypotonic fluid Fluid therapy: for electrolyte abnormality.
    • Low sodium diet.
  • Hyponatremia with hypo-osmolarity and decreased plasma volume.
    • Hypovolemia stimulates ADH release and thirst to promote retention of water.
    • Gastrointestinal losses (severe vomiting and diarrhea).
    • Third-space losses (peritonitis, pleural effusion, gastric).
    • Cutaneus losses (burns).
    • Hypoadrenocorticism Hypoadrenocorticism.
    • Diuretics.
    • Renal failure.
  • Hyponatremia with hypo-osmolarity and increased plasma volume.


  • If severe hyponatremia and the onset is acute then clinical signs develop quickly.
  • If hyponatremia chronic (over at least 2-3 days) potentially none or only minor clinical signs.
  • If correction of hyponatremia is too fast (more than 1 mEq/kg/h) neurologic signs of myelinosysis develop up to 3-4 days later.


  • None.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • De Morais H A & DiBartola S P (2008) Hyponatremia : a quick reference. Vet Clin Small Animal Practice 38 (3), 485-489 PubMed.
  • Schaer M (1999) Disorders of serum potassium, sodium, magnesium and chloride. JVEEC (4), 209-217 VetMedResource.

Other sources of information

  • The Veterinary ICU Book (2002) Eds W E Wingfield and M R Raffee. Teton New Media, Jackson Hole, WY.
  • Small Animal Critical Care Medicine (2008) Eds D C Silverstein and K Hopper. Saunders Elsevier, St Louis Missouri.
  • Fluid, Electrolytes and Acid-base Disorders in Small Animal Practice (2006) Eds S P DiBartola, Saunders Elsevier, St Louis Missouri.

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