Sodium Deficiency in Dogs and Cats (Hyponatremia)

Sodium deficiency, or hyponatremia, is more often than not a potentially life-threatening symptom of another underlying disease.

By: pkingDesign
Sodium deficiency can be life-threatening. By: pkingDesign

Sodium deficiency, or hyponatremia, can present in 2 ways. The first is a slow-developing lack of sodium with few signs, and the second is a rapidly occurring form with potentially life-threatening neurological symptoms.

However, the term “sodium deficiency” is a little misleading because this implies a lack of sodium in the diet. But the intake of salt (sodium chloride) is usually perfectly normal. Once in the body, salt becomes diluted, and key to the problem of hyponatremia is the concentration of sodium in the body.

Think of this in terms of making orange cordia: If you put an inch of cordial in the bottom of a tumbler and fill it with water, the drink is the right strength. If you put that same amount of cordial in a 5-gallon bucket and fill it up to the top, the drink is way too weak. In the case of very diluted sodium, this affects fluid in the brain and causes serious nerve symptoms.


If the concentration of sodium in the body falls gradually, then the dog or cat usually doesn’t show any signs because the body has time to adjust and work out a way to protect the brain from the changing levels.

However, if the concentration falls rapidly, the symptoms are potentially life-threatening.

In the early stages, the pet is lethargic, seems mentally dull and does not respond as he should.

This progresses to head tremors, seizures, possibly coma and then death.

Hyponatremia usually results from the presence of another disease. Therefore, the pet may well have signs associated with that underlying problem, such as a cough if the dog has heart disease, for example.


The story of hyponatremia is that of an underlying condition that dilutes the blood.

Perhaps the easiest of these conditions to understand is psychogenic polydipsia. This is a psychological condition where the pet drinks water compulsively. Taking in large volumes of water dilutes his blood and drops his sodium concentrations, like our measure of cordial in a 5-gallon bucket.

Other examples include drugs that cause fluid retention such as certain chemotherapy drugs, barbiturates and tricyclic antidepressants. Next on the list are diseases that cause fluid retention, and these include:

  • An underactive thyroid
  • Congestive heart failure
  • Nephrotic syndrome
  • Addison’s disease
  • Kidney failure

An unfortunate cause of hyponatremia is the too-rapid administration of intravenous fluids.

The other side of the coin is sodium loss from the body. This happens through severe vomiting, loss from the skin via burns and loss into a body cavity such as what happens with peritonitis or fluid around the lungs.


This is made by running blood tests and checking electrolyte (of which sodium is one) levels.

Also, if the concentration is low, it is important to screen for underlying conditions such as hypothyroidism and Addison’s disease. It some cases it may be necessary to run a CT or MRI scan to look for other causes of central nervous system (CNS) disease such as meningitis or a tumor.


Pets with sudden onset neurological symptoms, such as seizures, must have the fits controlled fast; diazepam is commonly used for this.

Then the blood sodium concentration can be slowly raised using intravenous sodium solutions. This must be done slowly — if sodium is added too quickly, the brain can go into shock, which is equally as dangerous.

For animals with low sodium concentrations but who show no symptoms, the best option is to manage the underlying disease reducing their sodium levels. For a dog with an underactive thyroid, this means starting an oral thyroid supplement; for a cat with nephrotic syndrome, it means a high-protein diet and using diuretics to shift retained fluid.


Low sodium concentrations usually develop as a result of disease. Most of these show symptoms such as increased thirst, poor appetite or change in body shape.

Thus, if you notice your pet is sick, seeking prompt veterinary advice is the best way to prevent complications such as hyponatremia.


  • “Hyponatremia — a quick reference.” De Morais & DiBartola. Vet Clin Small Animal Practice, 38: 491–495.
  • Fluid, Electrolytes and Acid-base Disorders in Small Animal Practice. Publisher: Elsevier.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Dec. 17, 2018.