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).
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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