Introduction
Hyperthyroidism (thyrotoxicosis)
is a common hormone disease of older cats that we have
diagnosed
with increased frequency in recent years. It is almost always caused
by a benign tumor of the thyroid gland that increases the amount of
thyroxine (the hormone secreted by the thyroid gland) in the
bloodstream. This increase in thyroxine causes an increase in the
overall metabolism of the body, leading to problems for several
internal organs. Even though this disease can be diagnosed in young
cats, most cats that get this problem are older. Cats 8 years of age
or older should be screened for this problem when routine blood
panels are run.
Cats that have Feline
Hyperthyroidism commonly have other problems that need careful
attention if the thyroid problem is to be treated successfully. The
excess thyroxine can cause these other problems, or make them worse
if they already exist. Some of these other common problems are
kidney
disease, heart disease,
dental
disease, sugar
diabetes (diabetes
mellitus) and high
blood pressure
(hypertension) leading to
blindness.
Just to show how unique each
species is, dogs usually get hypothyroidism,
the opposite problem with the thyroid gland.
Anatomy
The thyroid is a small and paired gland located at
the neck. If enlarged it can sometimes be palpated.
This
picture shows the right thyroid of a dog. Note its location
adjacent to the trachea (windpipe).
Physiology
The thyroid gland produces thyroxine, a hormone
involved with the bodies metabolic rate.
Pathophysiology
The benign nodules that appear on the thryoid
gland in this disease secrete excess of amounts of T3 and T4. In most
cases both glands are enlarged. These hormones are not under the
control of TSH (thyrotropin) secretion.
Cause
It is caused by a benign tumor (called an adenoma)
of the thyroid gland in almost all cases. This tumor produces excess
amounts of thyroid hormone, which circulates through be bloodstream
and affects the metabolism of many internal organs.
In rare cases a malignant tumor called a carcinoma
is the cause.
Symptoms
The symptoms that occur depend on
which internal system or systems are most influenced by the increase
in thyroxine circulating throughout the bloodstream. The more common
ones are:
- Weight loss
- Excess appetite
(polyphagia)
- Decreased appetite
(anorexia)
- Nervousness
- Muscle weakness
- Vomiting (emesis)
- Diarrhea
- Excess drinking and urinating
(polyuria and polydipsia)
- Poor hair coat
- High heart rate
(tachycardia)
- Labored breathing
(dyspnea)
It is easy to overlook some of
these symptoms, especially if they are subtle. Some people even think
of these symptoms as a normal part of the aging process of cats. If
left untreated hyperthyroidism can cause heart failure.
Diagnosis
The symptoms of this disease mimic
other common diseases in cats. These include kidney
disease, diabetes
mellitus (sugar diabetes),
cancer, and liver
disease.
Signalment
Signalment
This problem occurs almost
exclusively in middle aged and older cats. There is no know breed
or sex disposition.
History
Cats with this problem will
exhibit some of the symptoms noted above
Physical
Exam
During a physical exam some
cats will appear thin, have racing heart rates, and even heart
murmurs (click here
if you would like to hear what a murmur sounds like). Sometimes we
hear a specific type of heart beat called a gallop rhythm. Some
cats even have thickened nails.
In some cats we can feel an
enlarged thyroid gland. You might notice our doctors palpating
your cat's neck and throat during an exam to check for this
enlargement. Some cats can have an enlarged thyroid gland that can
not be palpated because it has slipped from its normal position at
the throat and repositioned itself further down the chest.
Diagnostic
Tests
An important tool in the diagnosis of Feline
Hyperthyroidism is a blood
panel. We can sometimes detect
the effects of the excess thyroid hormone on the internal organs by running
a routine blood panel. This panel might show an elevation in the red and white
blood cells. It is not uncommon to find elevated liver
enzyme tests in cats that have
hyperthyroidism. If the liver problem is secondary only to hyperthyroidism,
it will usually resolve when we treat the hyperthyroidism.
In addition to this blood panel
we run a specific thyroid test called a T4. Frequently this is all
we need to make the diagnosis. Since thyroxine levels fluctuate,
we occasionally need more sophisticated tests to make an accurate
diagnosis.
A cat can have a normal thyroid test yet
still have hyperthyroidism. This is because the thyroid hormone level fluctuates
throughout the day or it might be early in the course of the disease. Also,
these older cats can have other problems, which can suppress the production
of the thyroid hormone. In these cats we will run a free T4 test.
Sometimes a T3 suppression test
is performed. In a normal cat, the administration of T3 orally
will cause the T4 levels to decrease, in a hyperthyroid cat they
will not decrease or will only slightly decrease.
The
arrows at the top point to the elevated liver enzymes
tests in this cat. Note the arrow on the bottom pointing
to the very elevated thyroid level.
After 2 weeks of treatment the thyroid
level (arrow at top) and liver enzyme levels (lower arrows) have shown
a significant improvement.
One of the most precise tests
to diagnose hyperthyroidism is a scan of the thyroid gland. Not
only is this test accurate in diagnosing the problem, it will let
us know if some of the abnormal thyroid tissue has repositioned
itself further down the chest. an additional benefit is its
ability to detect a malignant cancer of the gland in the rare
occasion that this occurs.
This is the scan of a normal cat. Both lobes of the thyroid gland are visible.
This
cat has Feline Hyperthyroidism. It is the more common benign
version, and is present in only one lobe of the
gland.
Treatment
There are three primary methods of
treating this disease. Each has its advantages and disadvantages, and
depends on your unique situation. One of our doctors will discuss
which option is best used in your case.
Radioactive Iodine
The treatment of choice involves the use
of radioactive Iodine to selectively destroy only the part of the thyroid
gland that has a problem. It has the advantage of a very high success rate
and does not require anesthesia.
It will also destroy the diseased thyroid tissue that has moved towards the
chest. The disadvantage is the fact that your cat must be boarded at the treatment
facility for up to 10 days after treatment to minimize radiation exposure
to others. Approximate cost is $700. This treatment is done at a special center,
and is available only on a referral by one of our doctors. The center that
performs the scan to make the diagnosis is the same place that will institute
this treatment.
Before your pet is treated with
radioactive iodine we will test its kidney function with Tapazole
to determine if kidney function will be adequate after the
radioactive iodine destroys the abnormal thyroid
tissue.
Tapazole
(Methimazole)
A relatively common treatment is the use
of a drug called Tapazole. It is inexpensive and easy to give to most cats.
Disadvantages are the fact that it never cures the problem, so a cat will
need to be on it long term, and occasional cats get side effects to the drug.
It is used in cats that have chronic
renal failure, a common problem
as cats age. In these cats the excess thyroxine circulating can actually be
an advantage to the kidneys because it increases the blood flow to the kidneys.
This makes them more efficient at removing waste products. If we completely
destroy the excess thyroid tissue with radiation or surgery, a kidney problem
that was under control can now become serious. In these situations we administer
a dose of Tapazole that decreases the excess thyroxine enough to make your
pet feel a little better, but not so much that it will exacerbate a kidney
problem that is being masked. It is a compromise in therapy because it is
an attempt to balance two problems that are occurring simultaneously.
Side effects to Tapazole
include vomiting (emesis), lack of appetite (anorexia) and
depression. They can be minimized or eliminated with proper pill
giving technique (the pill is bitter) or with an adjustment in
dose. We will monitor throid levels for several weeks after
instituting this therapy to arrive at the correct dose for each
cat. Every 6 months we will check the thyroid level to verify we
are giving the proper dose of Tapazole.
There is a topical version of
Tapazole that can be formulated by one of our pharmacies. It helps
minimize the inconvenience of giving an oral medication to a cat
every 12 hours. It is expensive, and at this point in time, does
not work as well as oral Tapazole.
There is another medication
used to treat hyperthyroidism called propylthoiuracil (PTU). It
has more side effects than Tapazole so it is not commonly
used.
Surgery
Surgery (thyroidectomy) is also
used to treat this condition, and can be very effective. It has
the advantage of a rapid and successful cure rate. Its
disadvantages are the need for anesthesia and its inability to
remove diseased thyroid tissue that is in the thorax.
Complications could include improper calcium metabolism,
hypothyroidism (too low a thyroid level), and parallysis of the
throat muscles.
Tapazole is commonly used prior
to surgery to minimize the symptoms associated with this disease
and make the cat a better surgical candidate.
Complications
If left untreated there are
significant complications that can develop. Blindness can occur due
to retinal detachment from high
blood pressure. Long term
kidney damage and non-stop diarrhea might also be consequences, along
with heart failure and death.
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