8975 McLaughlin Rd., Brampton, Ontario

Hyperthyroidism in Cats

Cats have two thyroid glands located in their neck; their purpose is to aid in the regulation of the bodies metabolic rate.

Hyperthyroidism is characterized by the overproduction of thyroid hormone, resulting in a subsequent increase in the bodies metabolic rate. Hyperthyroidism common in older cats and is rare in dogs; however, hypothyroidism is commonly seen in dogs and is rare in cats. Hyperthyroidism is usually caused by a benign tumor formation on the thyroid gland which causes excess hormone production, less than 2% of cats have a malignant tumor present. Unfortunately, hyperthyroidism affects many organs, especially the heart.

Risk Factors and Causes

Older cats are most likely to develop hyperthyroidism, currently, no environmental factors that could contribute to the development of hyperthyroidism in cats are known, other than exposure to higher than normal levels of dietary iodine. No specific breed is known to have a predisposition to developing the disease.

Clinical Signs

The average age of affected cats is 12 years old, only around 5% of hyperthyroid cats are younger than 10 years of age.

The most common clinical sign of hyperthyroidism are:

  • Weight loss (from increased metabolism)
  • Increased appetite
  • Restlessness
  • Aggression
  • Increased urination and water consumption
  • Vocalization (particularly at night)
  • Periodic vomiting or diarrhea
  • Unkept fur
  • Anorexia (late stage of the disease)

Two secondary complications of hyperthyroidism are hypertension (high blood pressure) and a form of heart disease known as thyrotoxic cardiomyopathy. Hypertension develops due to the increased pumping pressure and elevated heart rate due to thyrotoxic cardiomyopathy. Around 25% of cats with hyperthyroidism become hypertensive.

In thyrotoxic cardiomyopathy, the heart enlarges and thickens to meet the increased metabolic demands. Both cardiomyopathy and hypertension are potentially reversible with proper treatment of the disease.


The first step in diagnosis is to determine the blood level of one of the thyroid hormones, thyroxine (TT4). When TT4 is elevated above normal, diagnosis if confirmed.

Occasionally, a cat suspected of having hyperthyroidism may have a TT4 level within the upper range of normal. In these cases, a second test (either a free T4 by equilibrium dialysis or a T3 suppression test) is performed.


Treatment may begin with blood tests, urinalysis, chest X-rays, an ECG, or blood pressure measurement. These tests are required to evaluate the overall health of your cat so we may provide optimal treatment and minimize the risk of complications.


Surgical removal of one or both thyroid glands can be performed and highly effective. If only one thyroid gland is abnormal, then only that section may be removed. The only real risk of this treatment method is that cats with thyroid problems are usually geriatric so surgery itself poses a minimal risk.

Oral Medication:

Lifelong administration of methimazole can control overactive thyroid glands. The medication takes weeks to normalize thyroid hormones initially and may cause side effects such as lethargy, vomiting, anorexia, fever, liver damage, anemia, thrombocytopenia, and decreased white blood cell count. These side effects are rare, but it is best to monitor your cat closely after starting the medication and blood work should be performed every 3 to 6 months to ensure no side effects occur. Methimazole works by blocking the production of excess thyroid hormone, rather than destroying the abnormal tissue, therefore the medication must be given for life.

Radioactive Iodine:

A very effective way to treat hyperthyroidism is with radioactive iodine therapy (I-131). When an injection of radioactive iodine is given, it destroys the abnormal thyroid tissue without endangering other organs. Iodine therapy does not require anesthesia and eliminates the need for daily medication. Treatment requires 1-2 weeks of hospitalization at a veterinary hospital.

Prescription Nutrition:

Feeding an iodine limited diet (like Hills y/d diet) can in some cases clear up the clinical signs of hyperthyroidism and normalize thyroid hormone concentration. The food is not medicated, but rather contains a very small amount of iodine in it. For the diet to work, the cats must be fed ONLY the prescription diet, nothing else.

Is There a Cure?

Recurrence is rare unless treatment is discontinued or failed. With iodine therapy, recurrence is rare, but if a thyroidectomy is performed and some abnormal cells are missed, hyperthyroidism can return. If methimazole or thyroid diets are discontinued, hyperthyroidism will return.


Prognosis is generally very good for cats diagnosed with hyperthyroidism if treatment is pursued and maintained. There is a risk of secondary kidney disease developing as a result of hyperthyroid-induced hypertension, so it is important to constantly monitor your cat at home and at your vets office with regular check-ups and bloodwork.

McQueen Animal Hospital is proud to provide you with finest quality services in animal care in the Brampton region; we are located at 8975 McLaughlin Road, L6Y 0Z6 and are happy to answer any of your questions via phone at 905-455-7387.

Author: Jessica Wilkans, RVT at McQueen Animal Hospital

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