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Feline Hyperthyroidism, Part One: Symptoms and Diagnosis

By Dr. Laci Schaible

Feline hyperthyroidism is the most common endocrine disease in cats. It results from the excess secretion of the thyroid hormones.

What does the thyroid gland do exactly?

The thyroid gland is an organ located at the front of the cat’s neck. The only function for ingested iodine is for thyroid hormone synthesis. Thyroid hormones play a large role in controlling the body’s metabolism. Patients with hyperthyroidism tend to burn up energy too quickly. Hyperthyroidism also causes the heart to pump more quickly which can cause cardiovascular problems.

What causes feline hyperthyroidism?

The overwhelming majority of cats with hyperthyroidism have benign adenomas (growths) involving either one or both thyroid lobes. Only 1-2% of cats with thyroid carcinoma (thyroid cancer) have hyperthyroidism.

Since hyperthyroidism has only been recognized as a disease entity since the late 1970s, several possible explanations have emerged. Some studies suggest ingredients widely found in commercially available canned and dry cat foods may be involved. Environmental contaminates such as insecticides have also been implicated. Cat litter has also come into question. Although numerous studies are still ongoing, no one causative agent has been proven.

Know the signs!

While many pet parents think hyperthyroidism is a disease of older cats, this isn’t accurate. While hyperthyroidism usually affects cats that are greater than 8 years of age, it has also been documented in an eight month old kitten. There is no gender or breed predilection for this disease.

The most common clinical symptoms associated with hyperthyroidism include:

  • Increased appetite, yet some cats eat less
  • Skin changes: patchy hair loss, matted hair, minimal grooming
  • Increased thirst and increased urination
  • Weight loss
  • Hyperactive/difficult to examine
  • Dehydrated, wasted away appearance
  • Vomiting
  • Diarrhea/bulky, foul smelling stool
  • Depressed/weak
  • Aggressive behavior/easily stressed


The test for hyperthyroidism is often fairly simple in itself. It is a basic blood test (total T4) to check for elevated thyroid hormone. Many veterinarians have the ability to run it in house and get same-day results.

If the serum total T4 is elevated and clinical signs match, then hyperthyroidism has been verified. If the serum total T4 is within normal limits (or on the upper end of normal) and the patient has clinical signs suggestive of hyperthyroidism then a free T4 by equilibrium dialysis is the next step.

Widespread Effects

Cats diagnosed with hyperthyroidism should have a complete medical workup. This includes a complete blood count, blood biochemistry panel with electrolytes, urinalysis, blood pressure, and lateral whole body radiograph because of the number of ways hyperthyroidism can affect the rest of the body.

  • A mild increase in the packed cell volume may be seen
  • Live enzymes are often elevated
  • Potassium levels may be decreased and those patients should be supplemented
  • Approximately 20% of cats with hyperthyroidism have asymptomatic urinary tract infections
  • Hyperthyroidism can have a profound affect on protein metabolism and possibly on serum fructosamine concentrations, of importance to diabetic patients
  • Cardiac enlargement may be present in a number of patients
  • Systemic hypertension is common
  • Retinal hemorrhage and/or detachment should be evaluated
  • “Hidden” kidney disease

Hyperthyroidism and kidney insufficiency may be seen alone or in combination in geriatric cats. Hyperthyroidism does increase the kidney’s filtration rate and kidney blood flow causing the kidney's to work overtime. When the thyroid is restored to its normal state of function, some degree of kidney compromise can be "unmasked." Because kidney disease can be revealed after hyperthyroidism is controlled, many veterinarians recommend treating hyperthyroidism medically before selecting the permanent method of treatment.

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