Hyperthyroidism

Library

• Anal Gland Disease
• Cancer
• Canine Wellness Program
• Dental Disease
• Diabetes
• Heartworm Disease
• Hip Dysplasia
• Hyperthyroidism
• Hypothyroidism
• Intestinal Parisites
• Kidney Failure
• LVH Recommendations
• Skin Conditions
• Subcutaneous Fluids
• Vaccinations
• Vaccine Fibrocarsomas
• Vaccinations for Cats
• Vaccinations for Dogs
• West Nile Virus
• Dangerous Household Items
• Links

Hyperthyroidism

Hyperthyroidism is a common endocrine (hormonal) disease in middle age to older cats (mean age around 13 years). It is a rare condition in dogs and is usually associated with malignant thyroid glands. In cats the disease is most commonly associated with benign thyroid gland tumors.

The thyroid gland is located on the trachea (windpipe) just below the larynx (voice box). It consists of two lobes - one on either side of the trachea. The thyroid gland secretes hormones - most notably thyroxin (T4) and triiodothyroxine (T3) - that activate metabolic processes in the body, increase the body's use of nutrients, and can directly affect the heart making it pump faster and harder.

Hyperthyroidism is a progressive disease. Animals typically have ravenous appetites but lose weight, may be hyperactive, have diarrhea, vomit, and may have a poor hair coat. Some cats will have behavior changes including restlessness, increased vocalization and wandering and heat-avoidance activity. Occasionally, the disease will progress to the point where the animal may not eat and will be weak.

A veterinarian, upon examining a hyperthyroid cat, may notice weight loss since the last visit, low muscle mass, a fast heart rate, an abnormal heart rhythm, and a possible heart murmur. The veterinarian may or may not feel an enlarged thyroid gland. An animal suspected of being hyperthyroid should have a full blood screen done including measuring the animal's T4. Complete blood work can help differentiate hyperthyroidism from other diseases as well as check for concurrent diseases. A hyperthyroid animal will have an elevated T4. Sometimes an animal's T4 will fall into the "gray zone"; this means the T4 is elevated but is not high enough to definitively call hyperthyroid. In this case, a Free T4 by Equilibrium Dialysis (FreeT4ED) may be run or a nuclear scan may be performed. (The latter requires special facilities that are not available in most areas.) It may also be necessary to perform an ECG and chest x-rays to assess the effects of the thyroid hormones on the heart. Approximately 80% of hyperthyroid cats have enlarged hearts.

There are three basic treatments for hyperthyroidism, although new treatments are currently being researched:

1. Anti-thyroid drugs such as methimazole (Tapazole®) - this medication does not cure the disease but controls the excess secretion of thyroid hormone. The pros of this treatment include it's relative inexpense, availability, and effectiveness. The cons include its inconvenience (must administer a pill daily to twice daily), and possible side effects, including vomiting and anorexia. Improvement is usually seen within 1-3 weeks. A follow-up thyroid check is done 4-6 weeks after starting treatment or after adjusting the dosage. A follow-up T4 should be done yearly or if problems arise.

2. Surgery can be done to remove the thyroid gland. Although the surgery is typically a straightforward procedure, these animals are often anesthetic risks. It may also be difficult to tell which side of the gland is causing the problem (a nuclear scan can differentiate), if other important glands associated with the thyroid gland may accidentally be removed and there is a potential to cause an animal to become hypothyroid (low T4). Some studies have also shown that up to 85-90% of all cats that undergo a thyroidectomy will have recurrence of the disease in the remaining lobe of the thyroid gland within 2-3 years of the original thyroidectomy.

3. Radioactive iodine treatment. This is currently the state-of-the-art treatment for feline hyperthyroidism. It is relatively simple, safe, and is typically curative. The negative side is the expense (around $650-$900) and the need for the cat to stay housed at a special facility for 8-10 days until they have excreted the radioactive material. Currently this procedure can be done in Springfield, OR and Seattle, WA and most recently in the Beaverton and Clackamas areas of Portland.

The prognosis for a hyperthyroid cat with treatment is good unless there are other disease processes present (diabetes, chronic renal disease, cardiac disease) or if the thyroid tumor is malignant.