Hyperthyroidism

Hyperthyroidism is a condition seen in older cats, generally over 12 years of age, in which an excess of thyroid hormone is produced by the body. The usual cause is an enlargement of the thyroid gland. It is not known why this happens, but some environmental cause is suspected, since hyperthyroidism appears to be increasing in frequency.

CLINICAL SIGNS

The clinical signs of hyperthyroidism are related to the function of the thyroid gland, which regulates body metabolism. Affected cats have a rapid metabolic rate and are essentially burning themselves out. They lose weight despite an appetite that ranges from normal to ravenous.

The increased food intake can overload the gastrointestinal tract, causing soft stools or diarrhea. The heart rate is elevated, which can put a strain on the heart muscles; this can lead to heart failure if untreated.

Restlessness and excessive vocalization (crying) are also frequently seen. Some severely affected cats may actually be depressed with a poor appetite due to the metabolic derangements taking place.

DIAGNOSIS

The clinical signs described above are not specific to hyperthyroidism. A number of other disease processes can look very similar. Diagnosis is based upon the results of blood tests including a CBC (complete blood count), Chemistry panel, and blood thyroid levels. Chest radiographs (x-rays) may be taken if heart involvement is suspected.

There are two thyroid hormone levels that we typically test in cats. They are called T4 and Free T4. In some hyperthyroid cats, only one of these is elevated, so it is wise to look at both.

TREATMENT

There are three methods of treatment currently available for feline hyperthyroidism. Each has advantages and drawbacks which must be considered when deciding how to treat a particular cat.

The most common treatment method in current use involves giving a medication twice daily. The drug, methimazole, interferes with the thyroid gland's ability to manufacture thyroid hormone, so less hormone is produced and released to the body. The dose varies with the individual patient's condition, and may change over time in any one cat.

In cats with secondary heart complications, another drug, propranolol, is given to help protect the heart from the effects of the disease until the condition is stabilized. These medications are available in several forms:

	Tablet
	Chewable Treat

	Flavored Liquid
	Transdermal Gel

The effectiveness of treatment is monitored by checking the thyroid hormone levels at intervals and adjusting the dose of methimazole as needed to maintain them as close to normal as possible. Body weight and observation of the patient's appetite and activity level are also helpful. We schedule the first recheck for two weeks after the medication is started to ensure that the dose is correct for the individual patient, then every two weeks if necessary to find the correct dose.

Once the condition is stabilized, we recommend a recheck about every 3 months, at which time we test thyroid levels. We also check a Complete Blood Count to monitor any possible side effect of the methimazole on the bone marrow. If a problem is found, it is usually reversible by decreasing or stopping the medication. We may recommend repeating the Chemistry panel to keep tabs on the liver and kidneys, which are prone to problems in older cats.

THINGS TO WATCH OUT FOR WITH METHIMAZOLE TREATMENT

Þ Appetite - Within a week or two of starting medication, the ravenous appetite should return to normal. Some cats will eat less than normal amounts for a while as a mild temporary side effect of the methimazole. Call you veterinarian if your cat refuses to eat.

Þ Weight gain - This is normal and expected, but your pet should not be allowed to become overweight.

Þ Activity - The excessive activity due to the hyperthyroidism will diminish and it may seem that your cat is very lethargic in comparison. Bear in mind that older cats are normally not very active and look at the total picture before deciding that the condition is not normal.

Þ Skin rash - Some cats develop an allergic reaction to the methimazole that causes a moderate to severe rash on the face and head with a lot of itching. The reaction may disappear upon decreasing the dose. If not, the drug must be stopped and alternate methods used to treat the condition. Call us if your cat develops any unusual skin condition.

A newer treatment method that may be the best hope for the future is radioactive Iodine therapy, provided in our area by several facilities. The patient is given one dose of radioactive Iodine. Iodine is normally taken up by the thyroid gland and used in the production of thyroid hormones. The hyperactive tissue in the thyroid takes up more of the radioactive Iodine than normal tissue and is destroyed by the radiation. Since the radiation is very weak, only cells containing the radioactive particles are destroyed; the remainder of the cat's body is not affected.

The treated cats are mildly radioactive for a while, so they are kept in the facility for about 3 days, then sent home with strict instructions on how to handle them to minimize exposure of the family to the radioactivity.

This method has the advantage of being easy to administer and non-stressful to the patient; it is usually curative, so no further treatment is necessary. Although the initial cost is high, there is long-term savings because there is less follow-up care needed than with the other treatments.

Surgical removal of the thyroid gland involves some anesthetic risk for an ill elderly cat, but with modern anesthetics and monitoring methods those risks are kept to a minimum. Usually the hyperthyroidism is stabilized with oral medication before the surgery is performed. After surgery, the patient must take a thyroid supplement, since most or all of the thyroid gland is removed. This procedure is not the method of choice of most veterinarians due to a high rate of complications.

With good communication between you and your veterinarian, and periodic monitoring of your cat’s condition, she can be expected to remain happy and comfortable for months to years, depending upon other conditions of aging that may occur. Please feel free to call us with any questions or concerns you may have regarding hyperthyroidism and your cat.