Hyperthyroidism in Cats
Contributed by Southwest Veterinary Oncology, PLLC
Diagnosis and Clinical Signs of Hyperthyroidism in Cats
Hyperthyroidism is one of the most common endocrine disorders diagnosed in cats.  Hyperthyroid cats will often present with
weight loss, increased appetite, vomiting or signs of hypertension.  Thyroid testing will usually reveal an elevated T4 level
but some cats can have borderline bloodwork and still be truly hyperthyroid cats.  The vast majority of cats (90-95%) will
have a benign, functional thyroid adenoma that will produce excessive thyroid hormone but has no risk of metastasis.  A
very small percentage of cats will have a thyroid carcinoma which will also respond to treatment with I-131 although a larger
dose will be required and there is a chance of metastatic disease.  A significant number of cats will also be hypertensive.  
Hyperthyroidism in cats will often mask underlying renal disease (see below).  For this reason we recommend full bloodwork
and a urinalysis prior to treatment.

Treatment Options for Hyperthyroid Cats
Hyperthyroid cats can be treated surgically, medically or with radioactive iodine therapy.  Surgery has some risk associated
with it as these are older, often hypertensive patients in a hypermetabolic state.  When bilateral disease is present it also
carries the risk of damaging the parathyroid glands which can result in life-threatening changes in calcium homeostasis.  
Post-surgically, cats may require thyroid supplementation and close monitoring and adjustment of calcium levels long term.  

Medical management of hyperthyroidism involves the use of methimazole (Tapazole) to decrease the production of active
thyroid hormone.  It does not have any anti-tumor activity, but only controls the clinical signs.  Most cats will require
increases in their dose over time as the tumor will continue to increase in size, and it needs to be given daily for the
remainder of the cat’s life.  Approximately one out of six of cats will have gastrointestinal side effects, and CBC’s must be
monitored for evidence of bone marrow dyscrasias .  

Treatment with radioactive I-131 is the only option to specifically treat the dysfunctional thyroid tissue with minimal impact on
any remaining normal tissue.  

How I-131 Works
Treatment involves giving an IV injection of a radioactive form of iodine (I-131).  The only tissue in the body that actively
uptakes this iodine is thyroid tissue which is actively producing the thyroid hormone.  Any normal thyroid tissue that is
present is atrophied and inactive, therefore it is largely spared.  The I-131 will also be taken up by the ectopic functional
tissue.    

In order to verify that a functional mass is present, to measure the mass, and to identify any possible ectopic or metastatic
tissue a pre-treatment scan with technetium is indicated.  I-131 is a very safe drug with a low chance of overdosing the
patient; the greater concern is under dosing the patient resulting in the need for a second treatment.  The tech scan allows
us to accurately identify those cats that require a higher than average dose, and is particular useful in cats that have a long
history of treatment or are very difficult to manage on medical treatments.  In asymptomatic cats that are diagnosed on
routine bloodwork, a scan is not required if finances are a concern.
Technetium-99m Scan

To visualize a thyroid nodule, a dose of a very low strength radioactive tracer (Technetium-99m) is given intravenously and
a scan of the cat is obtained with a gamma camera.  The radioactive tracer will concentrate in any abnormal, functional
thyroid tissues and allows us to confirm the diagnosis, evaluate whether disease is unilateral or bilateral, roughly quantitate
the amount of abnormal tissue present, identify any ectopic sites and look for evidence of malignancy such as invasion or
metastasis to regional lymph nodes. All of these factors are used to determine the dose necessary to effectively treat the
patient.  

I-131 Treatment Protocol
The tech scan is performed on a Wednesday or Thursday after the initial appointment.  Patients will come in the morning,
meet with the veterinarian, and then have the scan done later that day.  The radiation oncologist will examine the patient as
well as previous bloodwork and urinalysis, and interpret the scan that day to assess for presence of abnormal tissue, size
and location of the thyroid mass(es) and determine the appropriate treatment dose to be given.  Due to the low radioactivity
of the technetium (half life of 6 hours), the patient will only need to stay overnight and can go home the following morning.  
The patient will return the following Monday morning to receive their I-131 treatment.  Since the radioactive strength of this
drug is significant (half life of 8 days), patients will need to stay in the hospital in a dedicated radiation ward while the drug
decays to safe levels for release to the public.  The average patient will need to stay in the hospital for 4-5 days and can
usually be sent home on Thursday or Friday of that treatment week.  The occasional cat will need to stay in isolation longer
depending on the dose given.  Starting on Wednesday of the week of treatment, cats are scanned daily to determine their
radiation level and when they can be released.

Radiation Safety
Since the pet is eliminating radioactive material during the treatment week, there is no visitation allowed for the owners
therefore we will call them daily with updates.  The pets are taken care of 24 hours a day by veterinary personnel trained in
radiation safety.  Any toys or personal objects brought in with the cat cannot be returned to owners and need to be
disposed of properly following state regulations.

Once the cat’s radioactive emissions drop to an acceptable level (set by state and federal agencies) the pet can be
released to the owner.  We advise keeping the pet indoors for two weeks following treatment.  During this time the owners
should not spend significant time with the cat on their laps or neck and should avoid contact with children and pregnant
women.  Contact with other pets is fine.  

Treatment Results and Follow-up
Greater than 90% of cats undergoing treatment with I-131 will become euthyroid within one to three months following
treatment.  Recheck T4 values at one month can be normal, low or elevated.  Recheck thyroid values at three months
should be within normal range.  A small number (2%- 8%) of animals will remain hyperthyroid and will need a second
treatment in the future.  There are also a small number (<10%) of cats that become hypothyroid and need daily
supplementation long term.

Hyperthyroidism and Renal Disease
Hyperthyroidism in cats can frequently mask underlying chronic renal disease.  True kidney function cannot be accurately
assessed until the cat is treated and has normal thyroid function.  A rough idea of kidney function may be identified by a
Tapazole trial for several weeks prior to I-131 treatment.  It is given as an option to owners that would not treat their cat if
they discover that underlying renal insufficiency is present.  The feline kidneys seem to be very sensitive to high and low
thyroid levels, so our goal is always to return them to normal.  Cats with thyroid levels that remain less than 1.0 mg/dl should
be supplemented.  We will obtain full bloodwork and a urinalysis prior to treatment (this can be done at either the family vet
or here during workup).  We recommend recheck bloodwork and urinalysis at one month and three months post treatment.  
Potential renal function issues will be discussed in detail with owners prior to starting treatment.  Mild renal disease and/or
hypertension are not contraindications to treatment with I-131.
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Last Updated 4/10/13