| Hyperthyroidism is a
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.
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
- Nervousness
- Muscle
weakness
- Vomiting
- Diarrhea
- Excess drinking and
urinating
Diagnosis
During a physical exam
some cats will appear thin and have racing heart rates and
even heart murmurs (click here
if you would like to hear what a murmur sounds like). 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.
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. In addition to this blood panel we run a specific
thyroid test which is frequently all we need to make the diagnosis. Since
thyroxine levels fluctuate, we occasionally need more sophisticated tests
to make an accurate diagnosis.
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 all these options with you
before instituting any treatment:
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.
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.
Surgery 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. Some cats can also get a problem maintaining
normal calcium levels after surgery.
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