Hyperthyroidism is the most common endocrine (glandular) problem in cats. The disease is caused by overproduction of thyroid hormone which can be due to hyperplasia (enlarged gland) or adenoma (benign tumor). Only 1-2% of hyperthyroidism is caused by thyroid cancer. The condition can be seen in middle aged cats, but 95% of the cats with hyperthyroidism are over 10 years old. The cause of the enlarged thyroid gland is not completely known.

The normal thyroid gland is located in the middle of the throat area on either side of the windpipe (trachea). Small amounts of thyroid tissue can also be present in the front part of the chest cavity. Normally the thyroid glands cannot be felt, but when they are enlarged an experienced veterinarian can palpate the left and right lobe of the gland. Both lobes are usually enlarged rather than only one. The parathyroid gland is often attached to each thyroid lobe. This gland regulates calcium and phosphorus metabolism, which is very important to body function.

The most common symptoms of hyperthyroidism include enlarged thyroid lobe(s), weight loss, increased appetite, vomiting, diarrhea, and sometimes defecation in unusual places. An unthrifty dry looking coat is another common sign. Palpation of thyroid lobes by the veterinarian in all cats over 7-8 years can sometimes pick up the disease even before the symptoms develop.

Complications of untreated hyperthyroidism include severe weight loss, heart disease, high blood pressure and kidney failure. Up to 85% of cats with the disease experience hypertension (high blood pressure). Without proper treatment and early detection, hypertension can lead to heart disease, kidney failure and blindness.

Diagnosis of hyperthyroidism includes blood tests such as Blood Profile and CBC, Urinalysis, and Thyroid Profile. Other testing may be indicated depending on the health status of the cat. It is important to remember that cats can have more than one medical condition at any given time.
 

There are 3 treatment options for hyperthyroidism:

1. Medication - Tapazole (methimazole) is the most commonly used drug and this is given twice daily. The drug is relatively inexpensive making it an attractive option. In a small percentage of cats the drug can have severe side effects after the first three months of use therefore patients who stay on Tapazole should be monitored closely. The drug blocks the production of thyroid hormone but does nothing for the cause of the problem. The cost for medication for this treatment option is approximately $20 per month. Laboratory testing as needed would be an additional fee.
2. Surgery - Surgical removal of the affected lobe(s) of the thyroid gland is the most recommended treatment. In most cases, no medication for the problem has to be given after surgery. It is a moderately difficult surgery to do and there is the potential that not all of the thyroid gland will be removed since some of the gland may be located in the chest cavity as described above. Also, 20% of the cats who have one lobe removed will develop the problem in the other lobe. When both of the Thyroid glands are removed,many cats have to take a Thyroid supplement so that they do not become Hypothyroid(too little). The cost of the surgery with anesthesia, antibiotics and pain medication is approximately $450.00 any is usually done at  Specialized Veterinary Surgical Hospital.
3. Radioactive Iodine - This treatment destroys all abnormal thyroid without affecting the normal thyroid tissue. It is reported to be 90-95% effective. If the patient is on a thyroid medication, it is recommended that this medication be stopped at least one week before the treatment is begun. This treatment option destroys all abnormal thyroid tissue in the body wherever it is located. The treatment is only done in approved facilities such as a veterinary teaching hospital and the pet must stay at least one week at the facility. Usually one treatment is enough to take care of the problem. The cost for this procedure is approximately $750 - 1000.00.

After any of the above treatments, the thyroid levels usually return to normal within two weeks. Routine follow-up blood testing is recommended every three-six months even after treatment.