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What is Hyperthyroidism?
Hyperthyroidism, by definition, is the excess production and secretion
of thyroid hormone (T3 and T4) by the thyroid gland. It is now
considered to be the most common endocrine disorder in middle to older
aged cats (average age 13 yrs). In cats, this is usually caused by a
benign thyroid tumor (adenoma or adenomatous hyperplasia) either on one
or both of the thyroid glands. Rarely (1-3% of cats), hyperthyroidism
is caused by a malignant thyroid tumor (carcinoma).
How do I know if my cat is affected?
The most common symptoms observed in hyperthyroid cats include weight
loss despite an increased appetite, increased thirst and urination,
restlessness or increased anxiety, vomiting, diarrhea, and changes in
skin or hair coat. There are blood tests to determine the amount of
circulating thyroid hormone in your cat.
Why is it important to treat?
Thyroid hormone is essential for fueling every cell in the body,
however, too much of a good thing can be detrimental on every body
system. Eventually, the body can not keep up with the increased
metabolic demands which can result in multiple organ failure when the
body can no longer compensate.
What are the current treatment options?
The goal of treatment is to reduce the amount of circulating thyroid
hormone in the body. There are oral medications that block the
synthesis of active hormone in the body, but this is not curative and
requires medicating the cat daily for life. Only 85-87% of cats respond
to oral anti-thyroid medications and up to 15% can have side effects
including vomiting, diarrhea, bone marrow changes, and skin lesions.
Surgery can be done to remove the thyroid gland(s), but healthy tissue
is also removed and the parathyroid glands are at risk for being
removed due to their close proximity to the thyroid glands. Aside from
being an increased anesthetic risk (older, highly stressed, and often
very skinny cats with rapid heart rates and often compromised kidneys),
there are many nerves in that region that have the potential for
damage. Often, both glands need to be removed (70-90%) and then these
animals need to have thyroid hormone supplementation. Radioactive
Iodine (I131) is the safest and most effective treatment modality
currently available with a success rate of 96-98%.
What is I131 and its advantages?
Radioactive Iodine is preferentially taken up by the diseased thyroid
gland (not by healthy thyroid tissue) and permanently destroys the
hyperactive tissue. There are no side effects on the rest of the body
and a reduction in thyroid hormone (and consequently, symptoms of the
disease) occurs almost immediately. It is given as a single injection
under the skin and unless your cat requires sedation for a regular trip
to the veterinarian, there is no sedation required. The success rate is
excellent (96-98%) with a single injection.
What are the risks of treatment?
A very small percentage of cats may develop hypothyroidism after
treatment (too little circulating thyroid hormone), but these cats
rarely require treatment (thyroid hormone supplementation). This is a
risk seen with all the treatment options available and is dependant on
how long the cat has had the disease and how long it takes for the
normal thyroid tissue to start functioning again. Older cats have an
increased risk of kidney disease despite concurrent hyperthyroidism. In
fact, hyperthyroidism may temporarily mask underlying kidney disease
and after these animals have been treated, the decreased kidney
function may become more apparent. It is important to keep in mind that
this is not a direct result of the treatment itself. Your veterinarian
may recommend a trial of oral anti-thyroid medication (since it is
reversible) to determine the risk of your cat with treatment. Only on
rare occasions would this preclude treatment because excess thyroid
hormone can directly impair the kidneys and eventually cause kidney
disease or failure.
Are there pre-treatment requirements?
Prior to treatment, we require a baseline CBC, super-chemistry panel,
urinalysis including urine specific gravity, and a T4 that has been
submitted to a lab (not done in-house). All lab work must be current as
of 60 days prior to treatment, except the urinalysis should be current
from 30 days. Additional thyroid hormone blood work may be done by your
regular veterinarian to aid in the diagnosis. If your cat has a heart
condition either primary or secondary to the thyroid disease, your
regular veterinarian may recommend a cardiac work-up (ECG,
echocardiogram, thoracic radiographs). We require that your animal is
stable enough to board in a hospital with minimum medical care (cats in
the nuclear ward will require minimal handling to reduce exposure of
personnel to radiation). We will be able to administer oral medication
or administer subcutaneous fluids twice daily if necessary.
What can I expect?
- You
will receive a packet of information in the mail regarding specific
details of the treatment itself as well as requirements for treatment
at our facility.
- The morning you
drop off your pet, you will meet with the Dr. and staff that will be
caring for your cat to address any questions or concerns and to review
the treatment procedures and discharge instructions.
- The
duration of stay will usually be 3 days. Families with small children
or pregnant household members or cats released with a higher
radioactivity level will require stricter guidelines upon discharge or
additional hospitalization. You will receive specific discharge
instructions in your packet.
- Due to
strict state and federal regulations, only licensed and trained
personnel will be allowed to enter the nuclear medicine ward.
Therefore, you will not be allowed to visit your cat during the
treatment stay. You will be contacted every day with an update on your
cat.
- If your cat is easily stressed
or fractious in nature, one dose of light sedation may be necessary to
administer the radioactive material to ensure your cat receives the
entire injection appropriately.
- A deposit will be required prior to ordering of the treatment as each dose will be ordered specific to your cats needs.
- Follow up bloodwork will be done by your regular veterinarian 30 and 90 days after treatment.
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