FELINE HYPERTHYROIDISM
The thyroid glands are located in the neck and play a vital role in regulating the body's metabolic rate. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in the metabolic rate. This is a fairly common disease of older cats. Although the thyroid gland enlarges, it is usually a non-malignant change (benign). Less than 2% of hyperthyroid cases involve malignant thyroid gland tumors.
Many
organs are affected by this disease, including the heart.
The
heart is stimulated to pump faster and more forcefully; eventually, the heart
enlarges to meet these increased demands for blood flow. The increased pumping
pressure leads to a greater output of blood and high blood pressure. About 25%
of cats with hyperthyroidism have high blood pressure.
What cats are more likely to become hyperthyroid?
Older cats are at increased risk for developing hyperthyroidism. Environmental and dietary risk factors have been investigated and may play a role in predisposing some cats to hyperthyroidism, though the specific mechanisms are not known. No individual breed is known to be especially at increased risk, although the Siamese appears to have a somewhat increased incidence of hyperthyroidism than other breeds.
What are the clinical signs?
The typical cat with hyperthyroidism is middle aged or older; on average, affected cats are about 12 years of age. The most consistent finding with this disorder is weight loss secondary to the increased rate of metabolism. The cat tries to compensate for this with an increased appetite. In fact, some of these cats have a ravenous appetite and will literally eat anything in sight! Despite the increased intake of food, most cats continue to lose weight. The weight loss may be so gradual that some owners will not realize it has occurred or the weight loss may be quite rapid. Affected cats often drink a lot of water and frequently urinate. There may be periodic vomiting or diarrhea, and the hair coat may be unkempt. In some cats, anorexia develops as the disease progresses.
Two secondary complications of this disease can be significant. These include hypertension (high blood pressure) and a heart disease called thyrotoxic cardiomyopathy. Hypertension develops as a consequence of the increased pumping pressure of the heart. In some cats, blood pressure can become so high that retinal hemorrhage or detachment will occur and result in blindness. Heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands. Both of these problems are reversible with appropriate treatment of the disease.
What causes it?
Some of the risk factors for hyperthyroidism have been defined above. A specific cause has not been identified. The possible role of dietary iodine continues to be investigated as a dietary influence on development of hyperthyroidism.
How is it diagnosed?
In most instances, diagnosis of this disease is relatively straightforward. The first step is to determine the blood level of one of the thyroid hormones, called thyroxine (or T4). Usually, the T4 level is so high that there is no question as to the diagnosis. Occasionally, a cat suspected of having hyperthyroidism will have T4 levels within the upper range of normal cats. When this occurs, a second test, called a T3 Suppression Test, is performed. If this is not diagnostic, a thyroid scan can be performed at a veterinary referral center or the T4 can be measured again in a few weeks.
What are my options for treatment?
Because less than 2% of these cats have cancerous growths of the thyroid gland, treatment is usually very successful. There are three choices for treatment; any one of them could be the best choice in certain situations. Many factors must come into consideration when choosing the best therapy for an individual cat.
Several tests are performed before choosing any form of treatment. These tests are needed to evaluate the overall health of the cat and predict the chances for treatment complications. Such tests include blood tests, urinalysis, and x-rays; if available, an EKG, blood pressure determination, and cardiac ultrasound can be performed.
The three treatment options for hyperthyroidism are:
1. Radioactive iodine. A very effective way to hyperthyroidism is with radioactive iodine therapy (I131). It is given by injection and destroys all abnormal thyroid tissue without endangering other organs. Treatment requires one or two weeks of hospitalization at a veterinary hospital licensed to administer radiation therapy. Sometimes, but not always, the expense can be greater than for the other options.
2. Surgery. Surgical removal of the affected thyroid lobe(s) is also very effective. Because hyperthyroid cats are usually over eight years of age, there is a degree of risk involved. However, if the cat is otherwise healthy, the risk is minimal. If the disease involves both lobes of the thyroid gland, two surgeries may be required, depending on the surgeon’s choice of procedures. In many cats, only one thyroid lobe is abnormal, so only one surgery is needed.
If surgery is the treatment method chosen, the cat is usually treated with an anti-thyroid medication for several weeks prior to the operation. During that time, the ravenous appetite should subside and the cat will probably gain weight. Some cats also have a very fast heart rate and high blood pressure; these problems can be managed with medication before surgery. After one to two weeks, another T4 level is measured.
The cat is generally hospitalized for one night following surgery and returns home feeling quite well. It should eat normally after returning home.
3. Oral medication. Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland. Some cats have reactions to the drug, but that number is fairly small (less than 20%). However, the side-effects may begin as late as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia. Methimazole does not destroy the abnormal thyroid tissue but rather prevents the production of excess thyroid hormone. Therefore, the drug must be given for the remainder of the cat's life. Periodic blood tests must be done to keep the dosage regulated. This type of treatment is appropriate for the cat that is a poor surgical risk due to other health problems or is exceptionally old. As stated above, it may also be used for a few weeks to stabilize the cat that is at increased surgical risk because of cardiac complications.
Recurrence of the disease is a possibility in some cats. It is uncommon after radioactive iodine therapy. When surgery is done, recurrence is possible if abnormal thyroid cells are left in the cat. The remaining cells will likely grow causing the disease to recur. However, this occurs less than 5% of the time and usually 2-4 years after surgery. Another possibility for disease recurrence is that one lobe of the thyroid gland was normal at the time of surgery so it was not removed. Then, months or years later, it becomes abnormal.
Is the prognosis good?
Many owners of cats with hyperthyroidism are hesitant to have radiation therapy or surgery because of their cat's advanced age. But remember, age is not a disease. The outcomes following both surgery and radiation therapy are usually excellent, and most cats have a very good chance of returning to a normal state of health.
Can it be prevented?
There are no preventive measures to adopt, but middle-aged and geriatric cats should all receive a complete physical examination by a veterinarian every 6-12 months. Special attention should be given to thyroid enlargement and the typical clinical signs of hyperthyroidism.
Nuclear Medicine for Pets
FELINE HYPERTHYROIDISM AND RADIOIODINE THERAPY
HYPERTHYROIDISM IN CATS is caused by benign tumors of the thyroid gland approximately 95% of the time. The remaining 5% or so of affected cats have thyroid cancer. The abnormal tissue functions independently of the normal thyroid regulatory mechanisms, and secretes too much thyroid hormone. The excessive hormone production causes many serious side-effects, such as weight loss, nervousness, liver and heart problems, and high blood pressure. Kidney damage usually occurs. Almost all cats have normal thyroid tissue that is dormant because of the over activity of the tumors. This normal tissue revives after radioiodine treatment, and the cat regains its normal thyroid function.
RADIOIODINE THERAPY is considered the best treatment for feline hyperthyroidism because it is very safe and stress-free for your cat, and does not damage the normal thyroid tissue. Anesthesia or incisions are not required, and greater than 90% of the cats are cured within 1-3 months after treatment. Very few cats (<2%) do not respond to the first treatment, but almost all of these cats are cured if a second treatment is given. There is no risk of damage to the parathyroid glands, which reside within the thyroid tissue and maintain your cat's calcium balance. These glands are frequently damaged during surgery for hyperthyroidism.
PREPARING YOUR CAT for treatment is easy. Because hyperthyroidism can mask pre-existing kidney disease (by causing high blood pressure, and other changes), we want to make sure your cat will have adequate function after treatment. For this reason, we ask that your cat be treated with the anti-thyroid drug, Tapazole®, until the thyroid level is normalized. Once this is achieved, the kidney values should be checked. If they are within acceptably normal limits, your cat should be a good candidate for radioiodine therapy. We are happy to help you manage your cat's medication to help stabilize him and get him ready for treatment. If your cat cannot be medicated, or does not tolerate Tapazole®, or cannot be stabilized on a reasonable dosage of medication, we can still treat him, but we will take some special measures to help make sure no kidney problems arise. Since hyperthyroidism causes ongoing kidney damage while masking the true kidney function, it is important that these steps be followed when possible.
WHEN WE SEE YOUR CAT FOR THE FIRST TIME we will perform a thorough physical exam and take a complete medical history. We will need a copy of his medical records for review. You can bring them with you, or have your referring doctor FAX or mail them. We will discuss any other relevant health problems with you prior to treatment. If your cat will be admitted for therapy at the first visit, as most of our patients are, we will ask you to bring enough of his usual food to last for 7-10 days. We don't want to upset him by changing his diet while he stays with us. You can also bring a small toy or other favorite item for him to sleep with, but we cannot return this to you.
A THYROID SCAN is an important part of your cat's treatment plan, and is done first. An injection of a short-acting radioisotope, technetium, is given. The technetium is absorbed by functioning thyroid tissue. A very specialized nuclear medicine instrument called a gamma camera is used to take pictures of your cat, and all the areas of technetium absorption can be seen. This allows us to tell if your cat has benign or malignant tumors, and makes it possible for us to calculate a very individualized dose of radioiodine for each cat. Cats that appear to have thyroid cancer are treated with a higher dose of radioiodine, and usually respond well.
THE TREATMENT PROCEDURE is very simple for your cat. The calculated dose of radioiodine is given as a subcutaneous injection, just like a vaccination. There are no side-effects from the injection. It does not cause any vomiting, diarrhea, or hair loss, since the entire dose is absorbed by the abnormal thyroid tissue. State laws require us to board your cat after treatment until his radioactivity drops below a certain level. We begin checking the level about 5 days after treatment, and most cats can go home about 7 days after treatment.
WHILE YOUR CAT STAYS WITH US, he will be cared for 2-3 times daily. He will be in a climate controlled cats only room. The day length is regulated by an automatic timer, and we play the radio softly (cats seem to like classical music or talk radio best). We will call you every other day to tell you how he is doing, but you may call us more frequently if you like. We will call you the first day your cat is ready to go home, and set up a discharge appointment for you. Unfortunately, state law does not allow you to visit your cat during the boarding period, but we will do everything possible to make sure you cat has a pleasant experience.
AFTER YOUR CAT GOES HOME, you will need to follow some safety precautions for the first 2 weeks. We will give you a copy of the instructions at your first visit. These guidelines mainly cover how much direct physical contact you can have with your cat in the first 2 weeks post treatment, and how to dispose of cat litter. These guidelines are mandated by the state to prevent radiation exposure to people. Your cat will be slightly radioactive when he goes home, but there is no health hazard to people if the guidelines are followed.
DURING THE FIRST MONTH AFTER TREATMENT, you should gradually notice the symptoms of hyperthyroidism disappearing. Mainly cats begin eating and drinking less as their thyroid function normalizes. If you feel your cat isn't drinking or eating enough, or if you are worried about anything else, you should call us right away. About one month after treatment, your cat should have a physical examination. Blood should be drawn for a thyroid level (T4 test) and kidney tests (BUN and creatinine). You can have your referring doctor do this for you, or we can do it, as you wish. Please make sure we receive the results, so we can discuss them with you.
OUR GOAL is to cure your cat's hyperthyroidism, and give you the longest, happiest time possible with your cat. Because you have made a commitment to providing the best possible health care for your cat, we would like to offer whatever assistance we can to help you. We invite you or your referring doctor to call for consultation if your cat has any problems, even if they are not related to hyperthyroidism. Dr. Van Vechten, in her capacity as a board certified internist and oncologist (cancer specialist). will always be willing to discuss your cat's health.
Melinda K. Van Vechten, D.V.M., D.A.C.V.I.M.
(Oncology and Internal Medicine)
9700 Business Park Drive, Suite 403
• Sacramento, California 95827 •
(916) 231-0696
