Firefox - Safer, Faster, Better

Feline Hyperthyroidism

Randy Walker DVM

About Hyperthyroidism...

Hyperthyroidism is a condition in which the patient has an excess of thyroid hormone in the bloodstream. Hyperthyroidism occurs almost exclusively in cats (and people).

How Is Thyroid Hormone Produced And What Does It Do In The Body?

Thyroid hormone is produced by the thyroid glands which are two small glands located in the neck near the larynx (voice box). In the healthy dog, thyroid hormone is secreted into the bloodstream in steady amounts and (simply put) it regulates the patient's metabolism. A deficiency of thyroid hormone (called hypothyroidism) can lead to a slow metabolism while an overabundance of thyroid hormone (called hyperthyroidism) can lead to an overactive metabolism.

What Are The Symptoms Of Hyperthyroidism In The Cat?

The symptoms of hyperthyroidism can vary from mild to severe and will vary from patient to patient. The "classic" symptoms of hyperthyroidism are restlessness and hyperactivity, increased appetite, weight loss, increased thirst, frequent urination, frequent stools, behavioral changes such as increased vocalization and sometimes irritability, poor grooming and heart problems. Cats with hyperthyroidism will often show one or more of the above signs. Rarely are all of these signs noted in the same individual.

Do Dogs Get Hyperthyroidism?

Hyperthyroidism is rare in dogs.

Which Cats Are More Likely To Get Hyperthyroidism?

Hyperthyroidism most commonly occurs in cats over 9 years of age and affects both sexes with about the same frequency.

What Causes Hyperthyroidism?

Hyperthyroidism is usually caused by benign hormone secreting tumors of the thyroid glands or sometimes by cancerous thyroid tumors. Most hyperthyroid cats don't have cancer, however.

Specifically, What Effect Does Hyperthyroidism Have On The Heart?

Due to the prolonged stimulation of the heart by the thyroid hormone, the cat's heart muscle will thicken. This thickening of the heart muscle leads to smaller chambers within the heart and a loss of the heart's ability to pump blood efficiently (a condition called hypertrophic cardiomyopathy). This is a very serious and life threatening condition. Fortunately, this heart problem will often reverse itself after the hyperthyroidism is controlled.

How Is Hyperthyroidism Diagnosed?

We diagnose hyperthyroidism through a blood test. We also routinely scan the heart (through echocardiography) to look for hypertrophic cardiomyopathy and treat it as indicated.

How Is Hyperthyroidism Treated?

The three main ways in which hyperthyroidism is treated are by (1) radiocative iodine (2) medication or (3) surgery. In this day and age the best way to treat most cases of hyperthyroidism is with radioactive iodine. In our area the most popular form of radiocative iodine therapy is with a procedure called Radiocat. Through the medical approach we give the cat medication in tablet form to curb the output of thyroid hormone from the thyroid gland. Through the surgical approach we surgically remove the thyroid glands (thereby taking away the source of the thyroid hormone) and then place the cat on thyroid supplement. Of these three approaches, the Radiocat method is favored as a first choice in most cases. The surgical approach is usually reserved for those cases in which the other approachs are not giving satisfactory results.

Briefly, Describe The Treatment Protocol For The Hyperthyroid Patient if Medical Therapy Is Chosen.

After hyperthyroidism has been diagnosed, we place the patient on a twice daily dose of a tablet medication designed to reduce the output of thyroid hormone. We then check the thyroid levels 3 or 4 weeks later and adjust the dosage of medication as needed. Once the proper dosage has been established we then check the thyroid levels every 6 months and adjust the medication as needed.

Why Should The Thyroid Level Be Checked Once Every 6 Months?

The thyroid level should be checked every 6 months because the amount of medication needed to maintain the proper thyroid level will vary according to various environmental and patient factors. It is not uncommon for the amount of medication required to change periodically.  

- Randy Walker DVM
Sun Lakes Animal Clinic
May 15,2004

For additional information contact

"Until one has loved an animal, a part of one's soul remains unawakened."
- Anatole France, 1921 Nobel Prize Speech -

Titlebar Image: The Gentle Doctor, 1937-38 by Christian Peterson (U84.179) - Iowa State University