HYPERTHYROIDISM IN CATS
Hyperthyroidism is a common disease in middle aged and older cats. It is caused by a small tumor, which is usually benign, located on the thyroid gland. This tumor is of thyroid tissue, so it produces thyroid hormones. The tumor itself usually does not cause problems but the excess amount of thyroid hormones in the cat's system does.
Thyroid hormones regulate the body's metabolic rate. When the thyroid level is too high, metabolism speeds up. Body temperature, heart rate, respiratory rate and calorie consumption all go up. The signs and symptoms of the disease usually reflect this increased metabolic rate. Affected cats usually have a voracious appetite (though some cats will be nauseous from the disease and show vomiting and poor appetite instead), but become very thin. They often have a poor, dry haircoat. Many show behavior changes - restlessness, crying or vocalizing more than usual, destructiveness or even aggressive behavior. Any time we see a cat over the age of five with weight loss despite good appetite, behavior changes or chronic vomiting, hyperthyroidism is one of the first diseases we check for.
Hyperthyroidism often causes heart disease. It also aggravates kidney and liver disease. We are very careful to monitor the status of these organs in affected cats. Chest X-rays are usually done before administering an anesthetic to any cat with hyperthyroidism, to look for heart enlargement. Sometimes hyperthyroid cats will have heart murmurs, hypertension (high blood pressure) or irregular heart rhythms. Click here for more information on Hypertension in cats.
There are three ways to treat hyperthyroidism. The first is with oral medication to suppress thyroid hormone production. The medication, called Tapazole or methimazole, is given either once or twice daily. The dosage must be carefully adjusted to keep the thyroid level in the normal range. Usually at least two follow up blood tests are needed to establish the correct dosage. Testing is then repeated about twice a year. The tumor often continues to grow slowly as the cat ages, so the amount of medication needed can increase as the cat ages.
Tapazole can cause problems in some cats. Many cats will have some stomach upset on the medication, especially for the first few weeks. Itching of the face can also be a side effect. Some cats will develop dangerous anemias or decreased white blood cell counts. A test called a CBC, or complete blood count, should be done periodically when a cat is started on Tapazole. If red or white blood cell counts drop, the medication must be stopped. Tapazole is also somewhat costly, especially if a cat requires a high dose of medication to keep the disease under control.
For younger cats in fairly good health, radiation treatment of the thyroid tumor is the best treatment option. The cost of this is usually about $1000. Radiographic iodine is administered with a simple subcutaneous injection, just as we would give a vaccination. The iodine is taken up by the thyroid gland, which used iodine to make thyroid hormones. The radiation in the iodine injection selectively destroys the tumor within the gland. Chest X-rays and blood testing are required ahead of time to ensure the cat is a good candidate for the procedure.
The cat stays at a special treatment facility for 3-7 days after radiation treatment while it is clearing the radiation from its body. The treatment is not hard on the cat except that it must be away from home for several days. Some cats become depressed or don't eat well during this time.
The third option for treatment of hyperthyroidism is surgical removal of the tumor. The surgery is expensive ($800-$1000) but eventually pays for itself by saving repeated blood testing and medication costs. The surgery is often done at a referral facility, such as a veterinary school, that has 24 hour intensive care monitoring. This is because the surgery disturbs the parathyroid glands, located next to the thyroid glands, which regulate calcium levels in the body. Calcium levels can fluctuate wildly for a day or two after surgery, which can cause heart problems if not carefully monitored.
Surgery is difficult if the tumor extends past the base of the neck down into the chest where it cannot be reached. The anesthetic required is also a concern, especially if heart or kidney problems exist. Once the first few days after successful surgery pass, however, complications are rare and the cat will be expected to have a normal lifespan thereafter. With radiation treatment being no more expensive and becoming more available, surgery is no longer the recommended option.
Most hyperthyroid cats are elderly, so many veterinary clients choose to use the oral medication to treat the disease. The pills are fairly tasteless and can usually be crushed up in some canned food. We strongly recommend radiation treatment for younger cats, as it is cheaper in the long run and very safe. It is also a good option for cats that cannot tolerate the Tapazole medication.
