Treatment Explanation


Treatment for Hyperthyroidism in cats:

There are three options for treatment of hyperthyroidism. All three are identical treatment forms to those for human patients with hyperthyroidism.

Oral anti-thyroid medications

    This medication (methimazole) blocks the production of thyroid hormones by the thyroid gland. This oral medication does not cure hyperthyroidism, and is usually required twice daily (lifelong) to control the disease. Methimazole can be useful in the treatment of hyperthyroidism in cats but it is not an innocuous drug. Regularly scheduled blood tests are required to adjust dosages and to determine if potentially harmful side effects are present. Owners frequently find that oral administration of this drug to their cat is costly and difficult over time.


    Surgical removal of the thyroid tumor (s) is performed under general anesthesia. This procedure usually results in a return to normal thyroid function for the cat though the risk of anesthesia must be given careful consideration. If both lobes of the thyroid gland are not removed, approximately 70% of cats will eventually develop a functional benign tumor of the remaining tissue, requiring additional treatment or surgery.

    Alternatively, removing both thyroid lobes during the same surgery increases the risks of disturbing calcium metabolism, which is governed by the 4 small, adjacent parathyroid glands. Because affected patients are usually geriatric, and under-conditioned, they must be monitored for post-surgical side effects including low calcium levels (hypocalcemia), and kidney dysfunction. They are commonly hospitalized from 2-5 days. To lessen anesthetic and surgical risk to the patient, a cat may be required to undergo medical therapy with methimazole until physical condition improves. Occasionally, hyperthyroid cats are found to have functioning thyroid tumors in the chest cavity, where surgery is not feasible.

Radioiodine I-131

    Of the three treatment options, radioiodine is considered by many to be the treatment of choice for most hyperthyroid cats. Overall, radioiodine provides a simple, effective, and safe cure for cats with hyperthyroidism. This form of therapy has been used successfully for over 50 years in human medicine, and over 20 years in veterinary medicine. It requires no anesthesia and can be offered to medically stable patients, regardless of their age!

    How does it Work?

    Thyroid function requires the uptake of the element iodine in the body in order to produce normal thyroid hormones. If a radioactive form (I-131) of iodine is administered to hyperthyroid cats, it accumulates in thyroid tissue wherever it occurs in the body. Thyroid tumors accumulate the greatest amount of radioactive iodine. Once inside the tissue, the radioactive iodine emits radiation, which destroys the overactive thyroid cells.

    The radioactive iodine not trapped in the thyroid is excreted in the urine and to some degree in the feces. The amount of radioactivity emitted by the compound naturally decreases by half, every 8 days. Thus, the radioactivity remaining in the cat's thyroid tumor tissue will painlessly dissipate on its own. Normal thyroid tissue tends to be automatically protected from the effects of radioiodine since the uninvolved thyroid tissue is suppressed and receives only a small dose of radiation. As an added patient benefit, there is no injury risk to the adjacent parathyroid glands. The residual (normal) thyroid tissue resumes full function within 1-3 months after treatment. An average of 95-98% of I-131 treated cats are permanently and safely cured with a single injection!

    Is it Safe?

    "Radioactive" iodine, despite its somewhat scary title, is considered the "gold standard" for safety and efficacy in treating hyperthyroid cats. I-131 administration is a safe and effective treatment for feline hyperthyroidism. This therapy has been successful in large numbers of cats, and the only recognized deleterious side effect has been hypothyroidism (underactive thyroid gland). This occurs in an extremely small percentage of cats and almost never requires specific treatment. The greatest risks are to the doctors and staff who work in the thyroid unit on a long-term basis. However, with stringent safety regulations, protocols, and monitoring, this form of therapy can be safe for cats and the caregivers!

Radiotherapy: What happens to my cat?

On or before the day of admission, you and your cat will meet with a veterinary medical specialist at PVESC. Your cat will be thoroughly examined and the medical records will be reviewed. The doctor will discuss any admission tests required (which can be done at your general veterinary hospital or on-site at PVESC prior to treatment) to ensure that radioiodine therapy is the best option for your cat. The results of any tests performed at PVESC will be discussed with you before proceeding with treatment. These tests can include:

    * A complete blood count

    * A thyroid hormone level (T4 or free T4) to an outside lab

    * Serum biochemistry analysis

    * Urinalysis

    * Blood pressure

    * Full body radiograph

    * Cardiac Pro BNP test

    * Ultrasonography (cardiac ultrasound to evaluate function if needed)

If your cat is judged to be medically stable, he or she will be admitted to the radiotherapy unit within 24-48 hours of your appointment. The unit is specially constructed for this use and houses only cats that are receiving radioactive iodine.

The quiet accommodations include "Southwest" decor and housing in roomy and cheery cat condos (by Snyder Manufacturing, Inc.). These condos have separate bathrooms and shelves for snoozing. The unit includes windows for natural lighting, music, and heated floors. The patients enjoy watching patrons of our bird and squirrel feeders.

Once the dose of radioactive iodine for your cat has been determined, it is injected painlessly under the skin (subcutaneous) exactly like a routine vaccination.

From that point on, your cat need do nothing else but sleep, eat and play while the radiation dissipates to safe levels (usually 3-7 days). We like to spoil all our patients as much as safely permissible. This brief separation is likely to be harder for the owners than the patients!

Your cat will be monitored daily while in our care. By daily monitoring of your cat's radiation level, we can determine when this level has declined to that allowable by law. At this time, your cat can be released to you. You will be contacted daily with updates during your cat's stay in the radiotherapy unit. If you have questions or concerns, do not hesitate to call us.


Pre-Treatment Diagnostic Testing

Pre-Treatment Diagnostics

These diagnostic tests are required before your cat can be admitted for radioactive iodine therapy.  We recommend that these tests be completed at your regular veterinarian’s office one to two weeks before your radioactive iodine consultation.  If these tests have not been completed, they can be completed at Portland Veterinary Specialists at the time of your radioactive iodine therapy consultation.  

Lab work
      - CBC, Chemistry
      - SDMA
      - Urinalysis
      - Thyroid level

Blood Pressure

Whole-body radiograph    
      - Radiograph(s) can be emailed to PVESC if digital, or brought with you to your appointment

Admission Information


   1. You or your referring veterinarian may request an admission appointment.

   2. Anti-thyroid drugs (Tapazole, Methimazole, etc.) Should be discontinued 1-2 weeks prior to admission. Most other medications are allowable but should be discussed prior to the admission process.

   3. Food containing fish products should be discontinued 2 weeks prior to admission. Fish products have been found to prohibit the uptake of radioactive iodine.

   4. You are welcomed and encouraged to bring your cat's favorite foods and/or treats. We provide an ample, tasty feline menu as well.

   5. Cat toys may be kept with your cat but they cannot be returned.

   6. Unfortunately, the State of Maine, in accordance with the strict regulatory guidelines of the Nuclear Regulatory Commission cannot permit client visitation while cats are in the radiotherapy unit.

   7. Once admitted and treated with I-131, your cat cannot be released to you until his or her radioactivity levels drop to a specific range. In the extremely unlikely event that a patient dies from another illness while being housed in the I-131 unit, the remains must be held by us until radioactivity diminishes (eighty days).


Radioiodine therapy is considered the optimal treatment for cats with hyperthyroidism. It has an extremely high success rate and safety record and we are pleased to offer this state-of-the-art treatment.

The costs of therapy reflect costs associated with providing these services:

   1. Pre-admission consultation with a veterinary medical specialist

   2. Cost and administration of the radioactive iodine

   3. Hospitalization and patient care in the radiotherapy unit

   4. Litter, food, and patient monitoring with radiation monitoring equipment according to stringent state nuclear medicine regulatory guidelines

   5. Time and expertise of the staff

   6. Costs associated with nuclear regulatory licensing and adherence to strict safety guidelines for hospital personnel

   7. Radioactive waste-removal

The cost for treatment is $1500.00.  Pre-admission diagnostic tests are associated with separate fees.  These diagnostics may be completed at your primary care veterinary office or at PVESC and include such tests as radiographs, blood work (CBC Chemistry Electrolyte panel, T4, SDMA test), urinalysis and blood pressure.  Other tests, such as echocardiogram, may be recommended.  In the rare instance that your cat will require an increased dose of therapy, there will be an extra fee of $250.00 for cats that require more than 4 mCi of radioactive iodine for treatment due to the extra expense of the therapy and the extra hospitalization that is required. 


Admission Agreement


Owner: _________________________

Address: ________________________

Phone: (Day)____________________





Cat's Name: __________________

Age: _____ Breed: ______ Sex: ______

Color/Markings: ___________________



Referring Veterinarian: _______________________________________

Referring hospital/clinic: ______________________________________
Consent to Treat

I authorize Dr. Gail Mason to hospitalize and treat the above-described cat with radioactive iodine (1-131). I understand that my cat will remain at this facility (PVESC) after administration of radioiodine until the radiation levels have decreased sufficiently to permit release of my cat. Until this time, no visitation is permissible for human safety reasons. The radiation levels permitted are determined by the State of Maine radiation safety guidelines and regulations.

I understand that:

    * My cat will be medically evaluated (including blood/urine tests, radiographs, and ultrasonography) to assess overall health status and eligibility for treatment.
    * Though the radioiodine treatment is successful with one treatment 90-95% of the time, outcomes cannot be guaranteed. For any cats requiring re-treatment at PVESC, the cost will be one-half the original amount.
    * Rarely, a small percentage of cats (<5%) develop an under-active thyroid (hypothyroidism) within a few months after treatment. This situation would require daily thyroid supplementation.

In the event of an emergency, I authorize the veterinarians at PVESC to render such medical and/or surgical treatment as deemed necessary, and I accept financial responsibility for costs incurred.

I agree to follow discharge instructions that are provided to me and understand that pregnant women and children younger than eighteen should not have direct exposure to my treated cat for 2 weeks following hospital release.


Signature of Owner______________________________ Date_____________

Discharge Instructions

Discharge instructions for owners of radiotherapy-treated cats:

For the first two weeks please follow these detailed home instructions for handling your cat and litter waste.

Please note that if you cannot or will not follow these instructions, you must notify us, and we may need to keep your cat hospitalized for additional time before release (additional fees apply).

The thyroid hormone level continues to decline for 30 to 60 days after treatment. During this time, the symptoms of hyperthyroidism are expected to abate. Gradual weight gain and return to healthy body condition are expected. If your cat is showing signs of illness or depression, please contact PVESC.

Upon discharge from PVESC (after an average of 4 to 7 days after treatment), treated cats will still be excreting radioiodine in their urine, saliva and feces. This radioiodine is in a form that can be taken up by the human thyroid where it may cause damage. Even through the level of radioactivity is much lower than the level at which human patients are released from the hospital, you must exercise caution during this period. The remaining radioactivity will be gradually eliminated from the cat over the next 2 to 4 weeks.

1. Treated cats must remain indoors only for two weeks after discharge.

2. Pregnant women, children under eighteen and people with immune mediated diseases should not have any contact with the cat or litter pan for two weeks.

3. Prolonged close contact with your cat (under 3 to 6 feet) must be avoided during this time. Limit visits to 20 minutes per session. Visit and pet your cat briefly, but do not allow the cat to sleep on your bed with you. Avoid contact with urine and saliva and do not allow the cat to sleep on your bedding.

4. Foods containing fish products can be reinstituted post-treatment.

5. Use disposable litter pan liners and plastic gloves to minimize handling of litter/waste.

6. Wash your hands after handling your cat, its food dishes and litter pan.

7. There is no need to quarantine your cat from other pets in the household.

8. If your cat must be seen by a veterinarian before the end of the 2-week quarantine, please alert PVESC.

While the amount of radioactive material remaining in your cat’s body is low, it is prudent to follow the above instructions exactly. If this is not possible, please consider boarding your cat with PVESC during the quarantine period (additional fees apply).

Waste Disposal

Disposal of litter pan contents:

If your home is on a public sewer system:

  1. Use scoopable/flushable litter such a “World’s Best Cat Litter” or Swheat”

  2. Wear protective gloves (such as dishwashing gloves), scoop your litter pan twice per day and flush the waste down the toilet.

  3. At the end of the two-week quarantine, flush all remaining litter down the toilet. The litter pan and scoop can be washed with soapy water and flushed down the toilet; it’s not necessary to save these items for any extended period or to discard them.

  4. This is the approved method of the State of Nuclear Regulatory Commissions.

If your home has a private septic system or if you have a public sewer and choose not to flush it:

  1. Your cat’s litter box must be scooped twice per day.

  2. Wear protective gloves (such as dishwashing gloves) and place the waste in a Ziploc or tie bag, be sure to double-bag the litter and excretions, then place the bag in a plastic tote with a locking lid that has been lined with a large garbage bag. This tote should be stored outside and away from small children, other pets and wild animals.

  3. At the end of the two-week quarantine, add all of the remaining litter from the boxes to the trash bag in the outside tote, tie the bag and leave all waste until the 80-day mark on your calendar.

  4. The litter pan and scoop can be stored with the trash bag in the tote for 80 days. It can then either be thrown out or washed and reused.

  5. When it is time to discard the waste, simply remove the large garbage bag from the tote and dispose of it as you would your household trash. The tote itself may be disposed of separately or can be rinsed out and reused.

Follow Up

We would like to recheck your cat’s progress in 4 to 6 weeks. This includes a physical examination, thyroid level (T4), renal panel with SDMA, electrolytes and body weight. If you prefer to see your primary care veterinarian for this, please have the results forwarded to PVESC.

Waste Disposal

Disposal of Litter Pan Contents
If your home is on public sewer use flushable litter. To dispose of your cat's urine and feces during its first 2 weeks post-treatment, scoop the soiled litter daily and flush it down the toilet. This is the approved method of the State of Nuclear Regulatory Commissions. If you refuse to follow this method or, your home has a private septic system, you must take these steps:

   1. For the first 14 days after your cat returns home from receiving I-131 therapy, put on your gloves, use your litter scoop to drop all soiled litter into a Ziploc (or similar) bag. Zip it shut. Place this bag in the second Ziploc (or similar) bag and zip it shut.

   2. Place the double-bagged litter, feces and urine in a large Tupperware (or similar) type container, lined with a trash bag, and close it with a tightly locking lid.

   3. Follow this process for 14 days, placing all double-bagged litter, feces and urine in the Tupperware (or similar) container, and lock the lid after each addition to the container.

   4. Mark on your calendar 94 days after the date of discharge from our facility for the stored litter to be discarded.

   5. During the time you store the container, place it outside where it cannot be reached by small children, pets, wild animals, etc, or in a basement or garage. Do not place it in occupied areas.

   6. At the end of the second week, put on your gloves and, in one step, pick up the edges of the litter liner containing any remaining litter, tie them together and place it with the litter you have collected over the previous two weeks. Dispose of your gloves, and wash or dispose of your litter scoop in the outside trash. If you have been unable to successfully use a litter pan liner, dispose of your litter pan as well. You may now return to your normal litter disposal routine.

   7. 94 days after discharge from our facility, open the container, pick up the trash bag lining it, and place the bag in your outside trash. Do not bury the litter or use it in the garden. You may dispose of the container separately.

Patient Follow-up

What will Treatment be like for my cat?

The ideal goal of 1-131 therapy is to restore normal thyroid function with a single dose of radiation without permanently damaging normal thyroid tissue. Most hyperthyroid cats treated with 1-131 are cured by a single injection-No surgery! No anesthesia! No medication!

Successful treatment results in normal thyroid hormone levels within 2 weeks of treatment in 70-8O% of cats. Over 90% of treated cats reach normal hormone levels within 3 months post-treatment. Cats often feel better within days of treatment and most owners can expect gradual and steady health recovery within 2 months.


Medical Follow-up

Copies of all pertinent medical records and test results regarding your cat's treatment will be forwarded to your primary care veterinarian. We recommend a recheck examination, thyroid (T4) level and kidney (renal) profile with your veterinarian 2-3 months after I-131 treatment. Please have the results forwarded to PVESC.