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Name
Hyperthyroidism, Feline
Short Description
Thyrotoxicosis, Multinodular toxic goiter
Affected Animals: This is a common disease of older cats, with an
average age of 13 years and a general range of four to 20 years.
Cats of all breeds and both sexes can be affected.
Overview: Feline hyperthyroidism is the most common
hormonal abnormality and is a common disease of aging cats.
Hyperthyroidism is a feline disorder affecting multiple body systems
due to an overabundance of the thyroid hormone, which increases the
metabolism. Because of this increase, the cat's body systems
function at an accelerated rate that it may not be able to sustain.
As a result, problems with the heart, gastrointestinal tract and
kidneys may occur; weight loss, hyperactivity, and increased
appetite are classic signs of the disease.
Courtesy of: Conery Calhoon
The thyroid glands are located on each side of a cat's trachea. The
gland on the left is a normal size. The thyroid gland on the right
side of the trachea is noticeably enlarged.
Normal Thyroid Gland
Parathyroid Gland
Parathyroid Gland
Enlarged Thyroid Gland
Routine laboratory tests will diagnose hyperthyroidism, which can be
treated very successfully through medication, surgery, or
radioactive iodine therapy. Cats with severe complications of the
disease may not respond well to treatment.
Clinical Signs: The clinical signs of hyperthyroidism demonstrate
an overall increase in metabolism and involve multiple body systems.
The majority of cases will show signs such as weight loss,
polyphagia, hyperactivity, unkempt haircoat or alopecia, vomiting,
diarrhea, polydipsia, polyuria, and aggression. Less than 10 percent
of hyperthyroid cats will present with atypical signs such as poor
appetite, anorexia, lethargy, and weakness. Common physical exam
findings include an enlarged thyroid gland(s), cachexia, heart
murmur, tachycardia, and hyperactive behavior.
Symptoms: The common symptoms that most owners notice
include weight loss, a voracious appetite, and increased activity
level. Other common symptoms include vomiting, an unkempt haircoat,
patches of hair loss, and increased water intake and urination.
Description:
Hyperthyroidism is the most common hormonal
abnormality affecting cats. Most frequently afflicting cats eight
years of age or older, it is a multisystemic metabolic disorder
caused by high circulating levels of thyroid hormones. As a result,
the body's systems begin working at an accelerated rate that can
cause them to function abnormally. Most often, cats that develop
this condition have hyperfunctional thyroid gland nodules. In rare
cases, malignant thyroid tumors can lead to hyperthyroidism.
The elevated levels of thyroid hormones associated with
hyperthyroidism cause many of the body's functions to run much
faster than normal. Thyroid hormones affect the musculoskeletal,
cardiovascular, gastrointestinal, liver, urinary, nervous, and
behavioral systems. Common complications resulting from
hyperthyroidism include heart muscle enlargement, high blood
pressure, kidney function impairment, and intestinal problems.
Diagnosis:
Several other diseases, including chronic renal
failure, liver disease, and cancer, can have similar symptoms as
hyperthyroidism. A thorough history, physical exam, and routine
laboratory testing will lead to the diagnosis of hyperthyroidism in
the majority of cases. Routine testing includes a complete blood
count, or CBC, a blood chemistry profile, urinalysis, and a test of
the thyroid hormone concentration called a serum T4.
Cats with early signs of hyperthyroidism initially may show a
thyroid hormone, or T4, level that is within the normal range,
making diagnosis more difficult. Because hormone levels may
fluctuate between normal and increased early in the course of the
disease, it is important that cats showing early signs have their
thyroid hormone levels evaluated after a period of one to two weeks.
Sometimes more extensive thyroid tests, such as a free T4 or T3
suppression test, a thyrotropin-releasing hormone stimulation test,
or a radionuclide thyroid scan may be required. Veterinarians in
general practice may refer cats to a veterinary internist or
veterinary teaching hospital for some of these procedures.
Prognosis: The prognosis for uncomplicated hyperthyroidism
is excellent. If the cat's physical condition is extremely poor or
if another disease is present, the prognosis is not as good and will
depend on the response to treatment. If cancer has caused the
hyperthyroidism, thyroidectomy or radioiodine therapy may improve
signs for some time but will not cure the disease.
Transmission or Cause: The most common cause of hyperthyroidism is
independently hyperfunctioning nodules of thyroid tissue. More
rarely, malignant thyroid cancer also can cause hyperthyroidism.
There are no known risk factors for the condition other than aging.
Treatment:
Treatment of hyperthyroidism usually is rewarding
and extremely effective. The goal of treatment is to reduce the
circulating levels of thyroid hormones. There are three options for
treatment of hyperthyroidism: daily medications, surgical removal of
thyroid tissue, or destruction of the abnormal thyroid tissue
through radioiodine therapy. Of the three, only surgery and
radioiodine treatments can result in a cure. Medications only can
keep the disease under control while they are administered.
Before pursuing more aggressive treatment, many cats are treated
with methimazole to bring the thyroid hormone concentration down
into the normal range. Ruling out possible concurrent kidney failure
is essential before surgery or permanent destruction by radioiodine,
since permanent correction of hyperthyroidism can worsen renal
disease.
The most commonly used antithyroid drug prescribed to cats is called
methimazole. Most cats will need to be given methimazole twice daily
for the best results. The drug is introduced at a low dose and then
slowly increased to the most effective level for each individual
cat. Mild side effects associated with methimazole include loss of
appetite, vomiting, and lethargy.
Rare side effects include scratching to the point of causing lesions
on the face and neck, a low platelet count, and bleeding disorders.
These complications usually develop within the first few weeks of
treatment and may require stopping the drug. Contact the
veterinarian immediately if a cat on methimazole experiences any
side effects.
Drug therapy typically requires frequent veterinary follow-ups for
the first three months. Physical exams generally are scheduled every
two to three weeks to assess improvements of hyperthyroidism
clinical signs, especially of the heart, side effects of the drug,
and improvement in the thyroid hormone concentration. Some owners
may opt for long-term methimazole treatment if the cat responds
well, or fails to develop side effects or resistance to the drug.
Thyroidectomy is the surgical removal of the thyroid gland or
glands. If successful, this procedure will correct the
hyperthyroidism permanently, unless thyroid cancer is present, or if
only one gland is removed initially and the second gland continues
or starts to hyperfunction. Antithyroid drugs sometimes are used
preoperatively to reduce anesthetic and surgical risks and to rule
out concurrent kidney failure.
Possible post-operative complications, rare if the procedure is
performed by an experienced surgeon, include critically low
blood-calcium levels due to removal or damage to the parathyroid
glands, damage to the recurrent laryngeal nerve resulting in
laryngeal paralysis, and permanent hypothyroidism requiring
long-term thyroid hormone replacement.
Administered by injection, radioactive iodine treatment usually is
successful in curing hyperthyroidism and causes the lowest incidence
of side effects. However, it is not widely available because special
equipment is required, along with strict adherence to radiation
safety regulations. State and local radiation safety laws typically
mandate in-hospital isolation for eight to 12 days without owner
visitation.
Radioactive iodine destroys only functioning thyroid cells. Dormant
thyroid cells that do not take up the radiation are then able to
produce normal amounts of thyroid hormone at a later time. Very few
cats, less than two to four percent, will require a second treatment
or require thyroid hormone replacement therapy.
Prevention: There are no known preventive measures other than
monitoring cats for abnormal symptoms and taking them for yearly
veterinary examinations.
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