New England Regional Veterinary Imaging Center

Radioactive Iodine (I-131) Therapy

For more information please click here for a pdf brochure.

Injected painlessly under the skin, I-131 is absorbed throughout the body, including the thyroid gland.  Radioactive particles attack and destroy unhealthy thyroid tissue while normal thyroid and other body tissues remain unharmed.  I-131 has been shown to be over 95% effective in curing Hyperthyroid cats and people.

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What is Hyperthyroidism?

Hyperthyroidism is one of the most common diseases affecting middle-aged and senior cats.  It is a result of the enlargement of the thyroid gland, which in turn causes an increase in release of the thyroid hormones, T3 and T4. 
    Signs and symptoms of hyperthyroidism include:
    Increased appetite and/or thirst
    Frequent urination
    Weight loss
    Hair loss
    Behavior changes

If your cat is showing any of these signs, he should be evaluated by your veterinarian as soon as possible.   Your veterinarian can diagnose hyperthyroidism through a blood test.  A hyperthyroid cat will show an increase in the level of T4.  If your cat is diagnosed, it is important that he or she be treated before the disease begins to cause secondary, degenerative, or life threatening problems.

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Treatment Options:

Once hyperthyroidism is diagnosed, there are several treatment options you may consider:

    Medication (Tapazole)
    surgical removal of the thyroid (thyroidectomy)
    Radioactive Iodine therapy (I-131) 

Out of the three options, the safest and most effective, as evaluated by experts in medical treatment, is radioactive iodine therapy. 

This is the way it works is that the I-131 is injected into the cat subcutaneously.  It is then absorbed throughout the body, including the thyroid gland, where the beta particles attack and destroy the unhealthy thyroid tissue.  The normal thyroid tissue remains unharmed.  In approximately one month, the thyroid generally returns to normal function.

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Benefits of I-131 Therapy:

I-131 has been proven to be over 95% effective in curing hyperthyroid cats and people. 

    Does not require anesthesia in compromised cats
    Does not involve surgery
    No damage to healthy thyroid tissue
    No harmful side effects
    Most cost effective treatment (see graph)
    Eliminates daily medicating
    Results are seen within one month
    Recommended by virtually all medical experts

I-131 is the best and safest option owners have to treat their cats for hyperthyroidism.  Tapazole, although safe for most cats can have detrimental side effects.  Some hyperthyroid cats can have decreased gastrointestinal absorption, which can impair response to therapy with standard doses.  Liver disease, although infrequent, is also a concern for cats on methimazole.

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When considering a referral for I-131 therapy, please review the following protocol:

1. A Complete Blood Count, General Health Profile, and Thyroid level (T4) must be performed by your veterinarian within 30 days of therapy.  Chest radiographs and urinalysis, especially specific gravity, are also recommended.  Any further diagnostics such as ECG, thoracic, and abdominal ultrasounds should be performed by your veterinarian but upon request by your veterinarian, can be performed at our facility.  Patients on methimazole for longer than 90 days should discontinue the treatment at least 7 days prior to I-131 therapy and have a thyroid level done 7 days after discontinuing the medication.  Please contact our facility or consult with your veterinarian for a list of medications that may interfere with I-131 therapy.

2. Upon admittance, patients will be hospitalized in the nuclear medicine ward, separate from the rest of the hospital. 

3. The first day in the hospital we will place an intravenous catheter and administer Tc99, a radioactive isotope to measure the thyroid glands activity.  This will help us determine the proper amount of I-131 to administer to the patient. 

4. On day two, the patient will receive their dose of I-131 by a simple subcutaneous injection.  This is quick and virtually painless for your pet. Patients receiving I-131 therapy will stay in the hospital for 4 to 6 days.  This insures us to monitor the patient and to insure their radiation level is safe prior to discharge.

5. During your pet’s stay, they will receive daily assessments by our team of doctors and veterinary technicians.  They will have daily radioactive levels done to assess the decline in radioactivity. Please feel free to contact our facility to check the status of your pet, but we ask that you do not visit with them until the day of discharge. These are Federal guidelines on radiation safety. 

6. Upon discharge you will receive instructions from our staff as well as guidelines to follow for the first 3 months your cat is back in your home.  Your veterinarian will also receive medical records from our staff so they can better assist you in your cats post I-131 treatment and monitoring.  It is vitally important that any diseases that your cat may have acquired due to the hyperthyroidism be kept in check by regular (every 6 months) examinations and diagnostics done by your veterinarian.

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Our compassionate staff represents years of nuclear medicine experience and feline care.  We can promise that your cat will be in good hands at Myhre Equine Clinic / New England Regional Veterinary Imaging Center.


Researched and written by Joshua VanHoose and Diane Sickles, veterinary technicians at Myhre Equine Clinic and New England Regional Veterinary Imaging Center.