Feline Hyperthyroidism

Feline Hyperthyroidism

Written by Dr. Lyndsay Baxter

What is Hyperthyroidism?

Hyperthyroidism occurs when thyroid glands produce too much hormone.  The issue is usually caused by an enlargement of one, or both, thyroid glands.  Typically the enlargement is benign, but can sometimes become cancerous.  We typically see the condition in cats greater than 7 years of age.


Thyroid glands are located in the neck, one on each side of the windpipe. Thyroid hormone affects the cat’s metabolism.  Cats with hyperthyroidism experience increased appetite, weight loss, faster heart rates, and high blood pressure.  The latter can cause secondary damage to the kidneys and the heart. Additional signs include:

  • On and off vomiting and/or diarrhea
  • Increased vocalization or agitation
  • Decreased grooming (often a hyperthyroid cat will appear unkempt)
  • Patient may seek out cold areas in which to rest

How is my pet diagnosed with hyperthyroidism?

We’ll first conduct a physical exam to check the overall health of your cat. We’ll look for signs of weight loss, faster-than-normal heart rate, new heart murmur, thickened nails, unkempt hair coat, and a palpably-enlarged thyroid gland. During the exam, we’ll also look at your cat’s retinas to check for any abnormalities caused by an elevated blood pressure.

We’ll also take a small sample of blood and measure your pet’s blood pressure.  Both tests will give us a clear understanding of the extent and severity of the disease.  If your pet has a heart murmur, will recommend an ultrasound of the heart (echocardiogram) to determine if the murmur is caused by the thyroid disease or primary heart disease.

What is the next step after my cat is diagnosed with hyperthyroidism?

There are four main treatments.


Medication won’t cure the disease but will return the thyroid hormones in the blood to more normal levels. Felimazole (generic Methimazole) can be given in either tablet form or as a gel applied to the inner ear. Medication is usually administered twice daily for the rest of the cat’s life.  Side effects can include stomach upset (decreased appetite, vomiting, diarrhea) and bone marrow suppression.  Patients on thyroid medication are typically rechecked every 3 to 6 months to confirm that the disease is under control.

Surgical Thyroidectomy or Removal of the Thyroid Gland

Under anesthesia the veterinarian will remove the enlarged thyroid gland.  This procedure can completely cure the disease, but the patient must be a good candidate for anesthesia and surgery.  There is also some risk that the surgeon will not be able to remove all of the abnormal tissue and that the adjacent parathyroid glands may be damaged. .

Radioactive Iodine Treatment

Radioactive iodine treatment is considered the “gold standard” since the treatment is curative without requiring anesthesia/surgery.

Radioactive iodine is injected into the patient and is absorbed only by the thyroid gland.   The radioactive material will selectively kill the excess thyroid tissue without causing damage to the rest of the body.

The treatment must be performed at a specialized center. Your cat will need to remain at the center until the radioactivity in the cat diminishes (typically 3 to 5 days).

Dietary Management

Brookfield stocks a special diet (it’s called Hills Y/D) that severely restricts dietary iodine so that the thyroid gland cannot produce excess hormone. It’s important to note that your cat can eat nothing else for the rest of its life. If you are interested in pursuing this option, you can take home a can or two of the diet to see if your cat likes its taste and will eat it.

You’re Not Alone

You don’t have to decide which of the above options is best.  We’ll help you to decide based on our physical exam findings and the results of the diagnostic tests.

Hyperthyroidism is common in older cats.  Many pet owners and pets learn to live happily with it for years after diagnosis.  There are multiple support groups for pet owners who are managing hyperthyroidism and of course all of us here at Brookfield Animal Hospital are here for you.

For more information on hyperthyroidism in cats, please download this booklet from the Association of Feline Practitioners http://www.catvets.com/public/PDFs/ClientBrochures/Hyperthyroidism-B&W.pdf





Posted on

March 19, 2017

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