Hyperthyroidism in the Cat
This information is not meant to be a substitute for veterinary care.
Always follow the instructions provided by your veterinarian.
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
A diagnosis of hyperthyroidism is made when the level of thyroid hormone is
increased in the blood. Most hyperthyroid cats have very high levels of
hormone but some cats will have signs of hyperthyroidism with normal or only
slightly increased levels of thyroid hormone. Thyroid hormone levels can
vary over time so it may be necessary to check blood levels several times or
perform a different test called a T3 suppression test. The enlarged thyroid
gland(s) can often be felt in the neck. 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 nuclear medicine scan of the thyroid glands.
Both thyroid glands are abnormal in this cat and take up the
radioactive compound. The other black spot is the stomach lining
which will also take up the compound. |
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.
High levels of thyroid hormone may cause heart disease. 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. Cats with serious heart disease and
hyperthyroidism need to be treated for both diseases. The heart disease will
reverse in many cats after successful treatment of hyperthyroidism.
There are three types of treatment for hyperthyroidism:
- life long oral anti-thyroid medications
- surgical removal of affected thyroid glands
- treatment with radioactive iodine
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.
Methimazole may produce side effects in cats including depression, vomiting
and lack of appetite. These signs usually resolve without stopping the
medication. 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. 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.
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. Treatment
with radioactive iodine is only performed at selected specialty veterinary
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).
Radioactive iodine treatment of
hyperthyroid cats is available at
Washington State
University.
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.
Washington State University assumes no liability for injury to you or
your pet incurred by following these descriptions or procedures.
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Last Edited: May 29, 2008 8:32 AM
