Radioiodine Therapy

Radioiodine Therapy (131I) is the preferred treatment for feline hyperthyroidism. The treatment is safe, highly effective and permanent.  Radioiodine is given as one injection under the skin similar to a routine feline vaccine.


Appointments for radioiodine therapy consultations may be scheduled by calling (802) 863-2387. Consultations and treatments are available Monday through Thursday.

It is important to determine your cat is in good health prior to radioiodine therapy in order to avoid medical complications during the course of treatment.  Pre-treatment blood work (CBC, chemistry panel, urinalysis, total T4 level*) and whole body x-rays will be required. Blood work and x-rays must be evaluated within 4 weeks of radioiodine therapy. It is recommended that these be done in advance by your regular veterinarian in order to facilitate timely treatment.

We know how hard it can be to leave a beloved pet in an unfamiliar place. Our team will make every effort to make sure your cat is comfortable during their stay. Beds and background music are always provided. Appointments for radioiodine therapy consultations may be scheduled by calling (802) 863-2387.

If your cat is currently being treated with methimazole, this medication must be discontinued 7-10 days prior to the total T4 test and 7-10 days prior to treatment. This is to ensure their radioiodine dose will be accurate based upon their untreated thyroid level.


An Internal Medicine consultation is required for all clients interested in pursuing radioiodine therapy. Dr. Bryan Harnett will perform a physical exam, review your cat’s pre-treatment blood work and x-rays and address any questions you may have regarding radioiodine therapy.

Following this consultation, your cat’s treatment will be scheduled. If space is available, and pre-treatment tests are complete, clients may have the option of beginning radioiodine therapy on the same day as the consultation, Monday through Thursday.

Treatment & Hospitalization

Radioiodine therapy will be administered at Burlington Emergency & Veterinary Specialists (BEVS) by Dr. Bryan Harnett.

Prior to the radioiodine injection, your cat will be given a mild and safe intravenous sedative. The radioiodine is administered subcutaneously (under the skin) in one single dose. Your cat will then be placed inside their designated suite in our radioiodine isolation ward.

Radiation levels are monitored beginning the third day after treatment. The “cooling off” period, or the time your cat remains in the isolation ward, will vary depending upon their radiation levels. In an effort to limit radiation exposure to our clients and the environment, BEVS requires that our patients remain hospitalized for a minimum of 96 hours after treatment or until they emit no more than 0.5 mR/hr measured at one foot. Most cats can be released within 5-9 days.

Radiation safety regulations require that direct contact with hospitalized radioiodine patients be kept to a minimum. A web cam enables the hospital staff to carefully monitor your cat at all times.


When cats treated with radioiodine are approved for hospital release, they are emitting no greater than “background” radiation levels.  A person standing one meter (approximately 3 feet) from the cat will be exposed to radiation levels comparable to standing outside on a sunny day. Close contact (i.e. less than a meter) will result in slightly higher radiation exposure until residual radioiodine has been eliminated.

BEVS recommends that clients take the following precautions for a 2-week period following release:

  • Your cat should be kept indoors or in an enclosed area.
  • You should avoid prolonged physical contact with your cat (e.g. do not let them sleep in bed with you).
  • Always wash your hands after contact with your cat.
  • Rubber or latex gloves should be worn when cleaning the litter box and disposing litter.  Soiled litter should be disposed daily in a plastic bag and then placed in an outside receptacle.  Cats that share litter boxes may continue to do so.
  • Children less than 18 years of age or pregnant women should have no contact with your cat.

You may resume normal activities after this two-week period.

Please schedule a follow-up appointment with your regular veterinarian one month after discharge to recheck your cat’s blood work.


What is hyperthyroidism?

Hyperthyroidism is a common disease in middle-aged and geriatric cats caused by excessive thyroid hormone production. Ninety eight percent of cases of hyperthyroidism are due to a benign thyroid tumor (adenoma) or excessive thyroid gland growth (hyperplasia).  Malignant thyroid tumors are rare and account for only 1-2% of hyperthyroid cats.

What are the signs?

Common signs of hyperthyroidism include weight loss despite a good appetite, excessive water consumption, increased urination, hyperactivity, vomiting and diarrhea.  More serious signs include heart failure and hypertension.  Hyperthyroidism may be fatal if not treated.

Are there other treatment options for hyperthyroidism?

Treatment options for hyperthyroidism include daily oral or topical medication (e.g. Tapazole®), surgical removal of one or both thyroid glands and destruction of diseased thyroid tissue with radioactive iodine (radioiodine).  Radioiodine is considered the treatment of choice in the majority of cats with hyperthyroidism.

How does radioiodine treatment work?

In hyperthyroidism, the activity of normal thyroid tissue has been suppressed due to the excessive amount of hormone produced by diseased tissue. Radioiodine is concentrated exclusively by hyperactive thyroid tissue. With radioiodine therapy, the local emission of radioactive particles destroys the diseased thyroid tissue. Normal thyroid tissue and other organs do not accumulate radioiodine and are generally not affected by therapy. Thyroid hormone concentrations usually return to normal within one month of radioiodine therapy.

Is it safe for my cat?

Adverse side effects of radioiodine therapy are rare.  Because thyroid status influences kidney function, approximately 30-40% of cats treated for hyperthyroidism will see a mild to moderate increase in their kidney values on follow-up blood work. This increase is usually within the reference range and does not result in clinical signs of renal failure.

Less than 5% of hyperthyroid cats will become symptomatic for hypothyroidism (low thyroid hormone concentration) after radioiodine therapy. Signs of hypothyroidism include lethargy, obesity, dry flaky skin and matting of hair.  Hypothyroidism is not typically life threatening, but may require indefinite oral thyroid hormone supplementation.

If a cat becomes ill during the “cooling off” period following radioiodine therapy, radiation safety regulations limit diagnostic (e.g. blood tests) and therapeutic intervention. Therefore, it is very important that any other pre-existing disease be identified, and that the cat be eating well, prior to radioiodine therapy.

Is it effective?

Approximately 90-95% of cats with hyperthyroidism due to thyroid adenoma or hyperplasia are cured by a single treatment with radioiodine.   Cats not cured with this initial dose are generally cured with a second treatment.

Approximately 1-2% of cats with hyperthyroidism have a malignant thyroid tumor (carcinoma) and may not be cured with standard doses of radioiodine.

Can I visit my cat?

Without exception, direct owner visitation is not allowed while cats are in isolation.

Owners, however, are able to “virtually” visit their cats through our website. Each suite is assigned a webcam so you may view your cat at any time. If you do not have internet access, we are happy to show you live video images on our hospital monitor in our treatment area.

Should I bring anything for my cat while they’re hospitalized?

We want your cats to be as comfortable as possible while they’re away from home. Our hospital has both wet and dry food options, but please feel free to bring your cat’s food and treats from home. Please keep in mind that patients are fed twice a day, but we will do our best to accommodate feeding routines from home. Personal effects (pillow cases, small towels, t-shirts, small toys, etc) are welcome, however, these items cannot be returned due to radioactive contamination.


Virtual Visits

If your cat is currently undergoing Radioactive Iodine (131I) therapy for hyperthyroidism, federal law requires that they remain in strict radioactive isolation during their treatment. During this time period you are not allowed to have direct visitation with your pet. You can, however make use of our 131I Virtual Visit web cameras.

When you brought your cat in for treatment you should have been provided with a suite number. Click the suite number below and you will be taken to a web cam feed of the suite in which your pet is being cared for. Browsers that support the viewing site includes; Fire Fox and Internet Explorer 11. Chrome is not supported at this time.

If you were not provided with a suite number or you are having problems connecting to the video feeds, please contact us.

i131 Virtual Visit Suites


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