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What is hyperthyroidism?

Hyperthyroidism is the condition resulting from secretion of excess thyroid hormone.  In cats, this usually results from adenomatous hyperplasia or adenoma (benign tumor) of the thyroid gland.  Carcinoma (cancer) is a very rare cause of hyperthyroidism in cats.

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How can hyperthyroidism be treated?

Hyperthyroidism can be treated medically, surgically, or with radio iodine.  Medical treatment consists of administration of methimazole (Tapazole) one to three times per day.  Methimazole treatment will usually control hyperthyroidism but is not a cure (i.e., the drug will block thyroid hormone secretion but will not remove the thyroid tumor), and the drug must be given for the rest of the cat's life.  Surgery will cure the hyperthyroid condition and consists of the removal of part or all of the thyroid gland (the feline thyroid gland consists of two separate lobes).  Radio iodine (radioactive iodine) will also cure the hyperthyroid condition.  The procedure for this treatment is relatively simple, consisting of a single oral dose.

There is now a prescription diet available aimed to control hyperthyroidism by starving the thyroid of iodine. This in turn will restrict the thyroid gland’s ability to create the thyroid hormone. We recommend researching this diet by visiting the following article http://www.animalendocrine.com/wp-content/uploads/2011/01/Nutritional-Management-of-Feline-Hyperthyroidism-VMA-NYC.pdf

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Why would radio iodine be used instead of medicine or surgery?

Medical therapy will work for some cats, but there can be several reasons it may not be the best choice.  First, some cats are difficult or impossible to pill.  Second, mild reactions (e.g. loss of appetite, vomiting) are common, whereas a few cats develop serious adverse reactions to the medication (blood or liver problems).  Because of these side effects, periodic blood tests to monitor the cat's condition are necessary.  Finally, some owners may not want to have to medicate their relatively young cat for the rest of its life (pills must be given daily).

Surgery is generally an effective treatment for hyperthyroidism but may have disadvantages in some cats.  Many cats with hyperthyroidism have heart problems and are higher anesthetic risks.  There is also a risk that there will be temporary damage to the adjacent parathyroid glands during thyroid surgery resulting in hypocalcemia (low blood calcium).  This complication can be life threatening and result in extra hospitalization and cost.  After surgery, some cats will develop hypothyroidism and will require treatment with thyroid hormone pills for a period of time.   Finally, there is a small risk that the hyperthyroidism will not be cured with surgery or the condition will reoccur.

Radio iodine therapy has some distinct advantages over use of medical or surgical treatment.  With radio iodine, the need for anesthesia and the risk of hypocalcemia (the major disadvantages with surgery) are eliminated. Tapazole treatment is not needed; in fact, drug treatment must be discontinued for at least two weeks before radioactive iodine is given.  The major draw back is that after administration of radio iodine, the cat must be kept hospitalized for a period of 4 (four) days.

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How does radio iodine therapy work?

Iodine is an element required for normal health.  In the body, it is used primarily by the thyroid gland (located in the neck) to produce the thyroid hormones (T4 and T3 are the two major thyroid hormones).

Radio iodine hyperthyroidism is a form of iodine that has been made radioactive.  In it's radioactive state, it undergoes a natural process (decay) in which it gives off radiation.  The radiation given off consists of three types: alpha, beta and gamma.  The half-life of I-131 is eight days; in other words, one-half of the radio iodine goes through this process every eight days.

When taken into the body, a large percentage of radio iodine accumulates in the thyroid gland.  The remainder of the I-131 is excreted in the urine and feces. Once the radio iodine is taken up by the thyroid gland (or thyroid tumor in a hyperthyroid cat), the gamma rays and beta particles are released.  The beta particles are particularly lethal to the thyroid tumor cells.  The beta particles travel a maximum of 2-5 mm in tissue; therefore, beta-particles are locally destructive but spare adjacent hypoplastic thyroid tissue, parathyroid glands, and other cervical structures.  The radiation destroys the thyroid tumor cells and thus treats the hyperthyroid condition.

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How is the therapy given?

The radio iodine is given as a single dose on the first hospital day.  After the treatment is administered, your cat is placed in isolation.  In this case, isolation means keeping your cat in a separate "facility", away from other animals not receiving this treatment.  Your cat is then monitored over the next four days until the cat's radioactivity level is low enough to permit his or her return to you.

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How long is the hospitalization period?

The half-life of the iodine used is eight days.  The iodine is excreted primarily in the cat's urine.  The usual period is four days.

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How do I check on my cat's condition during treatment?  Is visiting allowed?

Due to the nature of the treatment, visitation is not permitted.  The cats are fed and monitored two to three times daily by a full-time veterinary technician and Dr. Marshall during their stay.  If you wish, you can call us to check on the status of your cat.

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Are there any side effects or risks of therapy?

Since the iodine is specific in its site of action, there is no hair loss or increase in skin pigmentation, as may be seen with other forms of radiation therapy (cobalt radiation).  Some cats seem to experience mild discomfort of the thyroid region (thyroiditis) at the beginning of therapy, but this resolves itself spontaneously and does not cause a problem.

Occasionally a cat will develop hypothyroidism (under-active thyroid gland) after treatment with radio iodine.  This is easily controlled with supplementation and may not be permanent.  Overall, side effects are extremely rare.

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What happens after the isolation is over?

You will need to wear latex gloves when scooping the litterbox. Much of the residual radioactivity will be eliminated through your cat's urine and feces.  Therefore, we recommend that your cat be provided with a litter box that has a liner and utilizes flushable style cat litter.  After changing the litter, your hands should be washed thoroughly.

Upon discharge, your cat will still be minimally radioactive. Even though the level of radioactivity is very low (much lower than the level at which human patients are discharged from the hospital) you should still exercise caution during this period.

All of the remaining radioactivity in your cat will gradually disappear over the next 2-4 weeks (through radioactive decay and excretion into the urine). Until this is complete, your cat will emit low levels of radiation. Because of this, we require that you also isolate your cat at home for one week further, two weeks if children are living in the household. This isolation can be any spare room in your home.

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What aftercare will my cat need?

The vast majority of cats require no specific aftercare.  Your veterinarian should perform a complete blood analysis after the two week isolation, then monitor the thyroid function three months after, and then on a six month basis as a part of their on-going health care plan for your pet.

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Does the radio iodine treatment always work to cure the hyperthyroidism?

Our studies have shown that a single dose of radio iodine is effective in curing hyperthyroidism in over 97% of cats that are treated.  Even those cats that are not completely cured after one treatment, however, show some lowering in their circulation thyroid hormone concentrations and improve clinically.

If hyperthyroidism persists for longer than three to six months after treatment, re-treatment with radio iodine is generally recommended to cure the disorder.  Virtually all cats that remain hyperthyroid after the first treatment are cured by the second treatment.

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Can the hyperthyroidism ever reoccur?

Yes, although it is very uncommon (less than 3% of cats treated).  In addition, such reoccurrences usually develop three years or longer after the hyperthyroidism was first treated.   Therefore, such relapses may indicate the development of a new thyroid tumor causing hyperthyroidism, rather than relapse from the first tumor that was treated with radio iodine.

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What do I do on the day of treatment?

Please bring your cat to the hospital at the scheduled time.  You should feed your cat on the day of admission to the hospital (fasting is not necessary).  Your veterinarian should have done all or the majority of tests necessary before treatment; if further tests are required, they can be performed by us but treatment may then have to be postponed.  If your cat has been on methimazole (Tapazole), this drug must be discontinued for at least two weeks prior to treatment.

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Should I bring anything for my cat?

If your cat eats a particular kind of food, we suggest that you bring a few cans so that we know exactly what you want your cat to eat during the time of hospitalization. Also, you will need to leave a carrier with us for the ride home as your cat is required to ride in one while on the way home. We will also need you to bring your cats x-rays.  Personal items (a sock or toy) to place in your cat's cage are not allowed due to contamination.

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What happens on the day of admission?

A full explanation of hyperthyroidism  treatment will be gone over with you.  If all of the other routine blood tests have been done by your veterinarian, we will treat your cat on the day of admission.

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Is radio iodine therapy expensive?

The cost of any treatment for feline hyperthyroidism is significant.  The cost for radio iodine therapy is generally a total fee of $1,200.00, but may be more if the hyperthyroidism is severe.  This includes the radio iodine itself, the cost of hospitalization, food, litter, and monitoring.  Tests required prior to therapy are not included.  This compares reasonably with most fees for surgical removal.  The cost of medication would average $400 to $500 per year plus the cost of repeat blood tests.

Please note, because we must order the dose of radio iodine before the treatment is administered, cancellation of a scheduled appointment (less than 48 hours before the appointment time) will result in a charge of $250.

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How do I pay for radio iodine therapy?

Payment is required at the time of service.  As far as method of payment is concerned - cash, checks or charge cards (Visa, MasterCard, or Discover) are accepted.

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Four Locations for Cats Radio iodine Treatment Centers

  • Greater Detroit Area:
    Dr. Dan Marshall (Director)
    Cats Veterinary
    43727 Gratiot
    Clinton Township, MI 49036
    (586) 463-9550
  • Southern Michigan and Northern Ohio:
    Dr. Jeff Dizik, Lincoln Park Veterinary Hospital
    3909 Fort Street
    Lincoln Park, Michigan 48246
    (313) 389-2222
  • Northern and North-Western Suburbs:
    Dr. Steven Bailey
    Exclusively Cats
    6650 Highland
    Waterford, MI 48328
    (248) 666-5287
  • Western Michigan:
    Dr. Tammy Sadek
    Kentwood Cat Clinic
    3215 Breton Rd. SE
    Kentwood, MI  49512 
    (616) 241-6369
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