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 made 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 or high blood pressure. It also aggravates kidney and liver disease. We are very careful to monitor the status of these organs in affected cats. We will check your cat’s blood pressure every three months in the first year after diagnosis, when it is most likely to become elevated (even if we are treating the disease). After that we check it every six months along with the thyroid level and kidney and liver tests. 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 or irregular heart rhythms.
There are now four ways to treat hyperthyroidism. The first is with oral medication to suppress thyroid hormone production. The medication, called 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 one follow up blood test is needed to establish the correct dosage. Testing is then repeated 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.
Methimazole can cause problems in some cats. Many cats will have some stomach upset on the medication, especially for the first few weeks. Some cats will develop dangerous anemia 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 methimazole. If red or white blood cell counts drop, the medication must be stopped. We will retest your cat’s blood about two weeks after beginning methimazole treatment, adjust our dose and retest again if necessary, and then monitor thyroid levels every six months after that.
Most cats will eat methimazole crushed in a little tuna or canned food. If your cat isn’t one of these and you are unable to give tablets we can have the medication mixed into a liquid suspension with tuna or chicken flavor. We can also have it made into a gel that you smear onto the ear to be absorbed through the skin. This is significantly more expensive than the generic tablets and is not absorbed as well as oral medication is. It’s a lot better than not treating your cat at all though!
A brand new option for treating hyperthyroidism is an iodine-restricted diet made by Hill’s. It’s called Y/D and it comes in both dry and canned versions. The thyroid gland uses iodine to make thyroid hormones. Less iodine intake means the gland can’t make as much hormone and the disease symptoms abate. The basic diet formula Hill’s used is similar to their G/D which is formulated for older cats, so Y/D will suit the many hyperthyroid cats who also have kidney disease or need a diet lower in sodium. It’s a good option for cats who are difficult to medicate and it will be less expensive than methimazole. Recheck blood testing is done 4 and 8 weeks after starting the new diet and then every six months as for methimazole.
The main difficulties with Y/D are getting cats to eat the food, since cats can be fussy eaters, and the fact that you cannot let your cat eat any other food or treats if the special diet is going to work. This includes flavored medications such as glucosamine supplements or gabapentin suspension and also any mice or other creatures your cat might catch in your basement. A single monthly dose of heartworm medication is fine but anything given daily is a problem. Any other food items are likely to have way too much iodine and will make the special diet completely ineffective. Hill’s has compiled iodine level studies of lots of products, such as Pounce treats and Pill Pockets, so we have a list of items we can and cannot use along with Y/D. You should assume that anything edible that’s not Y/D or a plain, unflavored medication is probably a problem.
Cats switching to Y/D who are already taking methimazole will get half their regular methimazole dose for the first two weeks and then methimazole can be stopped altogether. Most cats are getting their meds twice a day, so you would just cut to once a day for two weeks. If you are giving it once a day you would simply cut the dose in half. As with any food change you will want to switch gradually, so start the lower dose of methimazole once your cat is eating just Y/D.
Feeding just Y/D will be difficult for clients who own more than one cat if they are eating different foods. If you have more than one cat, they can all eat the Y/D unless another cat in the household is on a different prescription diet. Young cats should get a tablespoon or two of regular normal cat food every day to give them a little more iodine in their diet.
For younger hyperthyroid cats in fairly good health, radiation treatment of the thyroid tumor is a good treatment option. The cost of this is about $2000. Radioactive iodine is administered with a simple subcutaneous injection, just as we give vaccinations. The iodine is taken up by the tumor and the radiation selectively destroys it, without damaging other organs or tissues. Chest X-rays and blood testing are required ahead of time to ensure the cat is a good candidate for the procedure. This treatment seems expensive but eventually pays for itself by saving repeated blood testing and lifetime medication or prescription food costs.
The cat stays at a special treatment facility for 3-7 days after 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 University of Wisconsin veterinary school in Madison, Wisconsin Veterinary Referral Center (WVRC) in Waukesha, and RadioCat in Wheeling, Illinois all provide this service. The Fox Valley Referral Center in Appleton is another option. The private facilities in Wheeling and Waukesha offer better service, more available appointments (including Sunday drop off and pick up times), chest and abdominal ultrasound scanning included in the price, and more attention and loving care for the cats while they are there.
The last option for treatment is surgical removal of the tumor, but this has fallen by the wayside since radiation is safer and about the same price.
Our most typical hyperthyroid cats are elderly, so most of our 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 who cannot tolerate the methimazole medication. As mentioned, the new diet is now also an option that we can try as long as you are aware that we don’t have as much information on success rates and monitoring as with the other two choices.
Please let us know if you have questions about hyperthyroidism that this handout didn't answer. We are always happy to help you decide which treatment plan is right for you.