Overactive Thyroid or Hyperthyroidism in Cats

Frequently Asked Questions About Overactive Thyroid in Cats

Veterinarian doctor making a checkup of a cute beautiful cat. White background.
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Overactive thyroid in cats, or hyperthyroidism, is one of the most common endocrine conditions affecting cats, in particular, older cats over the age of 10. Some veterinarians estimate that about 2% of cats over 10 will develop hyperthyroidism. Due to factors that may include environmental exposures, the percentage of cats affected by an overactive thyroid is on the rise. Untreated, hyperthyroidism in cats can lead to heart failure or kidney failure and can be fatal.

This article looks at frequently asked questions about hyperthyroidism in cats.


The thyroid is a butterfly-shaped gland located on either side of your cat's windpipe. The thyroid produces hormones that regulate metabolism and organ function. When your cat is hyperthyroid, his or her thyroid becomes overactive, and produces an excess of thyroid hormone.

How Do Cats Become Hyperthyroid?

The main way cats develop hyperthyroidism is due to the development of a benign tumor, known as an adenoma, in the thyroid gland. The tumor secretes excess thyroid hormone, creating the condition of hyperthyroidism.

How Common Is It in Cats?

Hyperthyroidism is one of the most common endocrine conditions affecting cats. The median age for acquiring hyperthyroidism is approximately 13 years of age, and very few cats develop the condition before the age of 10. Some veterinarians estimate that about 2% of cats over 10 will develop hyperthyroidism, and, due to factors including environmental exposures, that number is rising.

Is Hyperthyroidism Dangerous?

Untreated, hyperthyroidism in cats can lead to heart failure or kidney failure and can be fatal.


Some of the common symptoms of an overactive thyroid in cats include the following:

  • Weight loss (typical, but not always)
  • Increased appetite without weight gain
  • Vomiting and diarrhea
  • Increased energy and friskiness
  • More vocalization
  • Demanding food more frequently
  • Drinking more water
  • More frequent urination
  • Decreased appetite (less common, but can be a symptom)
  • Decreased activity (less common, but can be a symptom)
  • Weakness (less common, but can be a symptom)
  • Labored breathing and panting (less common, but can be a symptom)


Primarily, diagnosis is made by a blood test, measuring the level of thyroxine (T4) in the blood. High T4 levels in a cat are considered indicative of hyperthyroidism. Occasionally, if results are not conclusive, a Free T4 test may be run. Some veterinarian will use other tests including T3 levels, thyrotropin-releasing hormone (TRH) stimulation test, and thyroid radionuclide uptake and imaging ("thyroid scans") to verify a hyperthyroidism diagnosis.

Treatment Options

The three conventional treatment options are antithyroid drugs, surgical removal of the thyroid, and radioactive iodine treatment to disable the thyroid gland. In some cases, a change in diet to a special low-iodine food may be able to resolve hyperthyroidism. Some practitioners also work with alternative therapies for milder forms of hyperthyroidism.

What Is Involved in Antithyroid Therapy for Cats?

Antithyroid therapy is the treatment of choice for many practitioners and cat owners, because it's non-invasive and inexpensive. Antithyroid drug therapy involves putting the cat on the drug methimazole -- brand name Tapazole -- a human antithyroid drug. The downsides are that in some cats, it does not resolve the hyperthyroidism, and giving the cat a pill daily for life may be difficult. A small percentage of cats have some lethargy and vomiting as side effects. The typical cost of antithyroid drug therapy is $25 a month for life.

What Is Involved in Surgery for Hyperthyroidism in Cats?

Surgery -- known as thyroidectomy -- removes the affected part of the thyroid gland.

Surgery can be an effective cure, and many veterinarians are capable of performing this surgery. Only a few days of hospitalization is required. The drawbacks, however, include the risk of anesthesia, particularly in an older cat, and the risk of removing the parathyroid glands, which can cause hypoparathyroidism. In some cases, there is also a risk of hypothyroidism if both lobes of the thyroid are removed. The typical cost of a thyroid surgery is around $1000,

What Is Involved in Radioactive Iodine (RAI) Treatment for Cats?

With radioactive iodine therapy, the cat receives a one-time injection of iodine I-131, which concentrates in the thyroid and irradiates and destroys the malfunctioning part of the gland. Healthy thyroid tissue is not damaged, and the risk of hypothyroidism is low. Almost all cats -- an estimated 98-99% -- receiving radioactive iodine return to normal thyroid function within a month or so of treatment. This procedure can be expensive, running approximately $1,200 to $1,500 on average. Some veterinarians are not set up to do this sort of treatment, as it requires about a week's isolation for the cat while the radioactive material clears their system and the cat and their waste products are again safe for human exposure. 

What Changes to Diet Can Help Feline Hyperthyroidism?

Hill's y/d Feline Dry and y/d Feline Canned cat food is very low in iodine. By reducing the cat's iodine intake significantly, which removes a building block for thyroid hormone, and may resolve hyperthyroidism in some cats. Learn more about the Hill's Y D diet here.

Alternative/Complementary Approaches

Some practitioners use alternative therapies for cats with mild hyperthyroidism. Because of the potential seriousness of hyperthyroidism, it's recommended that you work with a good holistic veterinarian or naturopath who specializes in pets to determine an effective treatment regimen for your cat. Some of the alternative medicine approaches that have been effective include:

  • Traditional Chinese Medicine, including herbs and acupuncture
  • Bugleweed/Lycopus: This herb may, if used for several days sequentially, have an impact on mild hyperthyroidism
  • Lemon balm/Melissa officinalis: Can in some cases reduce the thyroid's output of thyroid hormone and alleviate mild hyperthyroidism.