in Cats

What To Expect From The Vet

Hyperthyroidism in the Cat

Hyperthyroidism is the overproduction of thyroid hormone by the thyroid glands. Hyperthyroidism occurs most commonly in older cats and is rare in dogs. The average age of cats with hyperthyroidism is 13 years of age; only about 5 % of hyperthyroid cats are younger than 10 years of age.

There are 2 thyroid glands located in the neck. One or both of the glands can enlarge and overproduce thyroid hormone. Involvement of both glands is more common than involvement of one gland. Thyroid hormone affects the function of most organs in the body, so the signs of hyperthyroidism are quite variable.

Signs of hyperthyroidism can include:

·weight loss

·increased appetite

·increased activity and restlessness

·aggressive or "cranky" behavior

·a poor hair coat

·a fast heart rate

·increased water drinking

·increased urination

·periodic vomiting

·increased amount of stool or diarrhea

·occasionally difficulty breathing

·occasionally weakness

·occasionally depression

The average age of cats with hyperthyroidism is 13 years old.

A diagnosis is made by feeling enlarged thyroid glands, noting increased blood levels of thyroid hormone when blood work is done, or by a thyroid scan.

Testing for thyroid is pretty routine in most clinics when an older cat patient is presented.  And most hyperthyroid cats have very high levels of hormone so making the diagnosis is easy.

But, of course, there are exceptions; some cats will have signs of hyperthyroidism with normal or only slightly increased levels of thyroid hormone.
Thyroid levels are affected by sterids, stress, other diseases, and not only that, but thyroid hormone levels vary at different times of the day.  So your vet needs to be careful to avoid making the diagnosis too casually.  So, don't be surprised if it may be necessary to check blood levels several times or perform a T3 suppression test or other more sophisticated tests. 

Another complication is that  diseaeses of old patients are rarely simple; you can bet that there are several problems going on.

If the diagnosis is not obvious by blood tests, a nuclear medicine scan of the thyroid glands can be performed at certain specialty veterinary practices. The cat is given a small dose of a radioactive compound that travels by the blood to the thyroid glands. Hyperactive thyroid glands accumulate more of the compound than normal glands. After this test the cat must be hospitalized for a few days while it clears the radioactive compound from its body.

A blood panel and urinalysis are also performed to screen for abnormalities in other organs such as liver and kidney that may be present due to the advanced age of the animal.

An example of multiple problems going on at once is that high levels of thyroid hormone may cause heart disease.

In cats with hyperthyroidism, the heart may appear enlarged on x-ray or ultrasound and may show abnormal electrical activity on an ECG (electrocardiogram). Heart disease may cause fluid to build up in or around the lungs causing respiratory signs and trouble breathing. Cats with serious heart disease and hyperthyroidism need to be treated for both diseases. with a little luck, the heart disease will reverse in many cats after successful treatment of hyperthyroidism.

Treatment for hyperthyroidism includes:

Methimazole; a medication available in tablets, flavored suspensions, and transdermal pastes. (see note to your left about this inexpensive treatment)

If methimazole treatment causes excessive side effects or prove inadequate, then surgical removal of the thyroid gland and radioactive iodine treatment are available and often successful.

·life long oral anti-thyroid medications

·surgical removal of affected thyroid glands

·treatment with radioactive iodine

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The very cool picture to your left is a Technetium scan of a cat showing bilateral hyperthyroidism
Hyperthyroidism is a disease of old cats.  They usually are thin, scraggly, and almost emaciated sometimes, despite a voracious appetite.
Treatment with transdermal methimazole is fairly easy, inexpensive, and luckily is adequate treatment for many hyperthyroid patients.
Methimazole is available as a tablet, but it's much easier to apply the transdermal gel.  The ear is an ideal place to apply the gel as it is relatively hairless.
If methimazole causes excessive side effects or isn't working well enough, surgical removal of the cancerous thyroid gland is possible
All three treatments will reduce thyroid hormone levels and the signs of hyperthyroidism. Discuss the 3 options with your veterinarian. If your pet has other diseases, one treatment may be better for your cat than another.

The anti-thyroid pill is methimazole, also known as Tapazole®. Methimazole is given one to three times daily and must be continued life long. It takes several weeks for methimazole to reduce blood thyroid hormone levels to normal. If methimazole is discontinued, thyroid hormone levels will return to high levels over a few weeks. Methimazole may be used to reduce thyroid hormone levels to normal before surgically removing the thyroid gland(s).

Cats with heart disease may be too sick and fragile to anesthetize for surgery in which case methimazole can be given until the heart improves and the cat is stronger. Some owners (and their cats) find it difficult to give pills daily and may decide, after starting anti-thyroid pills, to later have their cat treated with radioactive iodine or surgery.

Remember that hyperthyroidism is a form of CANCER and methimazole is a "chemo" medication and it's not uncommon to see side effects in cats including depression, vomiting and lack of appetite.

Luckily, these signs usually resolve without stopping the medication.  But a more serious side effect is the development of low blood cell counts which are more likely to develop during the first 3 months of treatment. Blood cell counts should be evaluated every 2 weeks during the first 3 months. If blood cell counts decrease methimazole is stopped and another treatment method should be considered.

Surgery is not straight forward either; sometimes there is thyroid producing tissue ... often cancerous ... in places other than the usual thyroid glands in the throat.  And patients with high thyroid levels are way too sick and weak for surgery, so these patients need to be treated medically with methimazole and supportive care until they stabilize

Enlarged thyroid glands can be surgically removed. Methimazole is given for 1 to 2 months before surgery so that thyroid hormone levels are normal at the time of surgery. If both glands are enlarged, they can both be removed and most cats will still produce enough thyroid hormone by a few thyroid cells scattered through out the body to prevent hypothyroidism (abnormally low thyroid hormone levels). A few cats will become hypothyroid and may need to take thyroid pills.

Surgical removal of the thyroid gland(s) can usually be performed without complications. Occasionally complications may develop including damage to the parathyroid glands, which are closely attached to the thyroid gland, damage to nerves close to the thyroid gland or damage to the voice box.

Parathyroid gland damage causes low blood calcium that may cause seizures. Low blood calcium is treated with calcium or vitamin D. Nerve damage causes abnormal size of the pupils of the eyes and droopy eyelids. Damage to the voice box causes a change in voice.

I just want to emphasize that this is a delicate, potentially high risk surgery done on abnormal, cancerous tissue done on old patients with multiple problems.

Some cats will remain hyperthyroid after surgical removal of the thyroid glands. These cats have thyroid cells in abnormal locations, including inside the chest cavity where surgical removal is difficult.

This extra thyroid tissue is called ectopic thyroid. If you and your veterinarian decide that surgery is the best treatment option for your cat, a nuclear medicine scan (described in the diagnosis section) could be performed at a specialty veterinary practice before surgery to see if your cat has ectopic thyroid tissue.

If ectopic thyroid tissue is seen on the nuclear medicine scan, then a different treatment, either methimazole or radioactive thyroid treatment should be selected. Cats that have had surgery may have recurrence of hyperthyroidism. Blood thyroid hormone levels should be measured once or twice a year.

The very best treatment option for most feline hyperthyroid patients is radioactive iodine which is a treatment done only at a few specialty practices.

Radioactive iodine is given intravenously and will accumulate in the abnormal thyroid tissue killing the abnormal thyroid cells but sparing the normal thyroid cells. Radioactive iodine will also accumulate in ectopic thyroid tissue. Radioactive iodine treatment is very effective and rarely causes hypothyroidism. The cats do not have to be placed under anesthesia for the procedure.

The disadvantages of radioactive iodine treatment include the need to travel to a facility that offers this treatment and the need for the cat to remain hospitalized until the level of radioactivity decreases to a safe level as determined by the state radiation control office (usually 1 to 3 weeks).
And, of course, the treatment is relatively expensive.

Older cats with hyperthyroidism often also have kidney disease. Treatment of these cats is a delicate balancing act. Hyperthyroidism can actually improve kidney function by increasing blood flow to the kidneys. Some cats with kidney disease will show a worsening of kidney function after treatment for hyperthyroidism. Talk to your veterinarian about monitoring kidney function in hyperthyroid cats.

I hope I haven't frighten you by stressing the potential complications in treating hyperthyroid cats.  While all these potential problems are a reality, we vets have a lot of successful cases where you bring in a very sick, older cat that you love and we quickly diagnose the problems, stablize your cat with IV Fluid therapy and supportive care, start methimazole treatment with the end result of a remarkable recovery and a patient that lives happily for another few years.
If the thyroid cancer is extensive, specialized radiation therapy is available at specialty practices