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Feline Hyperthyroidism
Frequently Asked Questions, Information About Overactive Thyroid Conditions in Cats

by Mary Shomon

What is hyperthyroidism?

The thyroid is a butterfly shaped gland located on either side of a cat's windpipe. The thyroid produces hormones that regulate metabolism and organ function. With hyperthyroidism, the thyroid becomes overactive, and produces an excess of thyroid hormone.

How do cats get hyperthyroidism?

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

How common is hyperthyroidism in cats?

Hyperthyroidism is one of the most common endocrine conditions affecting cats, in particular, older cats over the age of 10. 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 that may include environmental exposures, that number is on the rise.

Is hyperthyroidism dangerous?

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

What are the symptoms of hyperthyroidism in cats?

  • 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)
How is hyperthyroidism in cats diagnosed?

Primarily, diagnosis is made by blood test, measuring the level of thyroxine (T4) in the blood. High T4 levels are considered indicative of hyperthyroidism. Occasionally, if results are not conclusive, a more definitive -- and costly -- test known as Free T-4 may be run. And, some veterinarian will use other tests including T3 levels, T3 suppression test, thyrotropin-releasing hormone stimulation test, and thyroid radionuclide uptake and imaging ("thyroid scans"), to verify a hyperthyroidism diagnosis.

What are the treatment options for hyperthyroidism?

The three conventional treatment options are antithyroid drugs, surgical removal of the thyroid, and radioactive iodine treatment to disable the thyroid gland. Some practitioners also work with alternative therapies for milder forms of hyperthyroidism.

What is involved in antithyroid therapy?

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. 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. Typical cost of antithyroid drug therapy is $25 a month for life.

What is involved in surgery for hyperthyroidism?

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. 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 approximately $900.

What is involved in radioactive iodine 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 cat receiving radioactive iodine will return to normal thyroid function within a month or so of treatment. This procedure can be expensive, running approximately $1,200 on average.

Many 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.

A national network, Radiocat, however, runs specialty radioactive iodine clinics for cats at 10 centers around the country, including: Phoenix, Ariz.; New Haven, Conn.; Wilmington, Del.; Marietta, Ga.; Wheeling, Ill.; Baltimore; White Plains, N.Y.; Pittsburgh, Pa.; Springfield, Va; and Waltham, Mass.

What alternative/complementary approaches may be pursued?

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.

Sticking Out Our Necks and this website are Copyright Mary Shomon, 1997-2003. All rights reserved. Mary Shomon, Editor/Webmaster
All information is intended for your general knowledge only and is not a substitute for medical advice or treatment for specific medical conditions. You should seek prompt medical care for any specific health issues and consult your physician or health practitioner before starting a new treatment program. Please see our full disclaimer.