Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Hyperthyroidism Is a Danger That Can Strike Your Adult Cat

Pet Column for the week of November 18, 1996

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
By Joseph Hahn
Information Specialist

The thyroid gland exists in all mammals. It is a bi-lobed structure in the neck produces thyroid hormone. The main function of thyroid hormone is to regulate the metabolic rate in the body. Thyroid hormone affects every cell in the body. Hyperthyroidism is the overproduction of thyroid hormone.

According to veterinarians at the University of Illinois College of Veterinary Medicine at Urbana, this is the most common endocrine disease in cats. It occurs in middle aged to older cats and has no breed or sex predisposition.

There are two causes for the development of hyperthyroidism in cats. Most commonly, the thyroid gland will enlarge and begin to produce excess thyroid hormone without spreading to other parts of the body. The second cause is a diffuse enlargement of the thyroid gland, generally producing less thyroid hormone, but traveling to distant parts of the body.

Veterinarians note that clinical signs are usually slow and gradual in onset, often taking up to a year to be noticed. The signs of hyperthyroidism are weight loss (most common), excess drinking and urinating, hair loss, diarrhea, vomition and hyperactivity.

Excess thyroid hormone turns up the cat's internal thermometer so they will be unable to tolerate heat and will seek out the coolest spot in the house.

Heart disease is often associated with hyperthyroidism. These cats have excessive heart rates and heart muscle changes that can lead to heart failure and death. These cardiac changes also lead to changes in breathing, such as panting and an inability to handle stress or exercise.

Diagnosis of hyperthyroidism can be done by checking baseline levels of thyroid hormone in the body. This test can be run easily in most veterinary clinics or at local laboratories. Veterinarians see 90-95% of hyperthyroid cats with abnormally high baseline levels of
thyroid hormone. For those cases that are suspected of having hyperthyroidism, but do not have abnormal levels of thyroid hormone, a thyroid suppression test can be performed to help confirm a diagnosis of hyperthyroidism.

There are three main treatments for hyperthyroidism:

1. Drug therapy - The drugs used inhibit the synthesis of thyroid hormone. There are many side effects of drug therapy and the medical treatment would be life long. This treatment is usually used in the short term until one of the following two therapies can be instituted.
2. Surgery - This option is highly effective and routinely done. Some of the concerns with this therapy are recurrence of the disease if only one of the lobes is removed, removal of the parathyroid gland (which is critical for calcium regulation in the body), and anesthetic risk for cats that have developed heart problems.
3. Radioactive Iodine - This is a newer form of treatment and consists of injecting radioactive particles that concentrate in the thyroid gland and are destructive to it. These cats need to be hospitalized about 5 days until their radioactive levels fall to acceptable levels. While this sounds dangerous, it is very safe, effective and non-stressful to the patient.

One side effect of treatment with surgery and radioactive iodine, is the development of hypo-thyroidism. Veterinarians agree that this is a minor complication that can be treated with thyroid supplementation that is readily available and extremely affordable.

Once this disease has been treated, and thyroid hormone levels return to normal, these cats can live normal and healthy lives. If you would like further information or would like your cat tested for hyperthyroidism, contact your local veterinarian.