You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Hyponatraemia is an uncommon laboratory finding, but there are several disorders that can cause it

Normal sodium concentrations in the blood are reported to be  :

• Dog - 140-155 mEq/l or mmol/l. (320-355 mg/dl)
• Cat - 145-157 mEq/l or mmol/l.  (355-360 mg/dl)

Sodium in the body is mainly concentrated in the extracellular compartment - and sodium movement is accompanied by fluid movements in the body. If sodium is retained water is retained. If sodium is lost from the body  in urine, water is lost as well.

False low concentrations of sodium will be found if the animal has hyperlipidaemia or hyperproteinaemia because of displacement of plasma volume with lipid or protein respectively. However, there are several different causes of low sodium concentrations in the blood including :

• Excessive sodium losses
  • Diarrhoea - if it causes severe dehydration
  • Diabetes mellitus - due to water retention in the blood and osmotic diuresis
  • Diuretic overdose
  • Hypoadrenocorticism (Addisons Disease)
  • Mannitol or dextrose intravenous fluids - osmotic effects
  • Renal failure - end stage
  • Vomiting - if it causes severe dehydration
• Overhydration
  • Acute renal failure - polyuric
  • Iatrogenic - excess intravenous fluid therapy
  • Psychogenic polydipsia  - rare
• Other conditions - which result in relative overhydration
  • Congestive heart failure - mechanism is controversial but probably includes :
    • Decreased renal pefusion
    • Low blood pressure
    • High BUN concentrations - increase diuresis
    • Over-zealous use of diuretics
  • High blood pressure 
  • Hypoalbuminaemia - nephrotic syndrome, cirrhosis
  • Post-trauma - water retention

NB Contrary to popular belief it is virtually impossible to induce hyponatraemia solely by feeding a diet that is relatively low in sodium because so little sodium  is actually needed to maintain circulating concentrations, and all complete prepared pet foods exceed the minimum dietary requirements.

Last updated : October 2013