Treatment Explanation

There are four options for the treatment of hyperthyroidism in cats.  Each of these treatments is identical to that used in human medicine, except for total dietary iodine restriction.

  1. Oral anti-thyroid medications: Methimazole (Felimazole®) blocks the production of hormones produced by the thyroid gland. This oral medication is usually required twice daily and does not cure hyperthyroidism; it only controls the disease.  Therefore, the drug requires lifelong administration. Although methimazole is useful in the treatment of hyperthyroidism, it is not an innocuous drug. Regularly scheduled blood tests are required to adjust dosages and to determine if potentially harmful side effects have occurred. Owners frequently find that oral administration of this drug becomes costly and difficult over time.  The incidence of side effects associated with methimazole ranges from 20-44%. These side effects vary from being mild and transient, to severe and potentially life-threatening.
  2. Thyroidectomy: Surgical removal of the thyroid tumor(s) is performed under general anesthesia. This procedure usually results in a return to normal thyroid function, though there is always an inherent risk when putting an animal under anesthesia.  To decrease anesthetic and surgical risk to the patient, a cat may be required to undergo medical therapy with methimazole until his/her physical condition improves. Occasionally, hyperthyroid cats are found to have functioning thyroid tumors in the chest cavity, where surgery is not feasible. In addition to the risk of anesthesia, if both lobes of the thyroid gland are not removed, approximately 70% of cats will eventually develop a functional benign tumor in the remaining tissue, requiring additional treatment or surgery.  Removing both thyroid lobes during the same surgery does increase the procedure’s long-term effectiveness; however, it also increases the risk of disturbing calcium metabolism, which is governed by the four 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 for 2-5 days to monitor these conditions.  Surgical treatment has been largely replaced with other, safer options unless the thyroid tumor is known or thought to be malignant in nature.
  3. A limited iodine diet: Over the last few years, a nutritional management approach has become commercially available for hyperthyroid cats.  This is a prescription diet with the product name Hill’s feline y/d diet. This diet essentially is devoid of iodine. Since iodine is only required in the body for synthesis of thyroid hormones, the production of thyroid hormones is inhibited. The diet must be “acceptable” to the individual cat being treated and NO OTHER food or treat may be given to cats being treated with this diet. The company reports that there may be a 4- 12-week lag time before the thyroid hormones are effectively reduced, which may limit its efficacy in cats who are significantly symptomatic.
  4. Radioactive Iodine (I-131): Of the four treatment options, radioactive iodine therapy is usually considered the treatment of choice for most hyperthyroid cats. Overall, radioactive iodine therapy provides a simple, effective, and safe cure for cats with hyperthyroidism (requiring just a single treatment in over 95% of all cats treated). This form of therapy has been used successfully for over 50 years in human medicine and nearly 30 years in veterinary medicine. It requires no anesthesia and can be offered to medically stable patients regardless of their age.

Normal thyroid function requires the uptake of elemental iodine into the tissue for hormone production. When a radioactive form of iodine (I-131) is administered to hyperthyroid cats, it accumulates in the thyroid tissue wherever it may occur in the body. Once inside the tissue, the iodine emits radiation, which destroys the overactive tumor cells.  Normal thyroid tissue tends to be virtually protected from the effects of radioiodine since the uninvolved thyroid tissue is suppressed and receives only a small dose of radiation.

The excess radioactive iodine is excreted in the urine and to some degree, feces. The half-life of radioactive iodine is eight days.  In other words, the amount of radioactivity emitted by the compound naturally decreases by half every eight days. Thus, the radioactivity remaining in the cat’s thyroid tissue will painlessly dissipate on its own. As an added patient benefit, there is no risk to the adjacent parathyroid glands. The residual (normal) thyroid tissue resumes full function within 1-3 months after treatment. On average, 95-98% of I-131 treated cats are permanently and safely cured with a single injection.


Radioactive iodine (I-131), 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 that boasts a 95-98% success rate with a single treatment.  This means the hyperthyroidism is corrected, and normal thyroid function will resume in 3 to 6 months.  Results for any one patient; however, cannot be guaranteed.  There is less than a 3% risk of hypothyroidism (underactive thyroid) resulting from radiotherapy.  Of those cats that become hypothyroid, less than half will become symptomatic and require treatment.  Those that do require treatment can be easily maintained on supplementation.  Radioactive iodine has NO effect on any other organ in the body besides the thyroid.  Post-treatment azotemia (kidney insufficiency) is always possible despite careful screening.  It is not; however, related to the iodine itself.  Each patient is screened as thoroughly as possible to detect other underlying disease.  Any diagnosed concurrent health issues are discussed with you prior to your cat’s admission for treatment.  Sudden death during the treatment period has been reported rarely.  In these cases, occult heart disease was the end diagnosis.  Any patient who presents with detectable cardiomegaly, an arrhythmia, or greater than a 2/6 systolic murmur is offered a cardiac screen to assess cardiac status.


On the day of admission, you and your cat will meet with a veterinary medical specialist at PVESC. Your cat will have a physical exam and his/her medical records will be reviewed. During the consultation, the doctor will discuss the results of the pre-admission testing required to ensure that radioactive iodine 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 pre-admission tests will include:

  • Complete blood count
  • Thyroid hormone level (total T4 and free T4 if warranted)
  • Serum biochemistry analysis including SDMA
  • Urinalysis
  • Lateral whole-body radiograph
  • Possible cardiac ultrasound to evaluate heart function

If your cat is determined to be medically stable, he/she will be admitted to the radioactive iodine therapy unit.  The unit is specially constructed for use with radioactivity and houses only cats that are receiving the I-131 treatment.  The unit is quiet, and accommodations include roomy cat condos with separate areas for the litter pan and sleeping quarters.  The unit has radiant, in-floor heating, soothing background music, and a large window with a view of our bird and squirrel feeder.  This window also allows for natural lighting.

Once your cat has been admitted, the radioactive iodine is injected under the skin exactly like a routine vaccination.  From that point on, your cat needs to do nothing but sleep, eat and play while the radiation dissipates to safe levels. We like to “spoil” all of our guests as much as is safely permissible. This brief separation is likely to be harder for the owners than the patients!  During his/her stay, your cat’s medical condition and radioactivity level will be monitored daily.  Once the radioactivity level has dropped to 0.50 mR/hr at 16.5” (1/3 meter) or below, your cat will be released to you at a scheduled pick-up time.  You will be contacted each day with an update on your pet and we encourage you to call during regular office hours if you have further questions or concerns.

  1. Your regular veterinarian needs to send a referral form and your cat’s records to PVESC.  Once we have received the paperwork, you may call to set up an admission consultation.
  2. If your cat is currently on an oral anti-thyroid medication, you need to discontinue that medication at least one week (preferably two weeks) prior to admission.  Please consult with your veterinarian about the possible side effects associated with discontinuing this medication.
  3. As a precaution, food containing fish products should be discontinued at least one week (preferably two weeks) prior to admission, as the iodine in fish may compete with and/or inhibit the uptake of the radioactive iodine.
  1. Diet:  If your pet has dietary constrictions, we recommend you bring in their food at admission.  The only diet items not allowed are those containing fish products and raw meat diets.  PVESC will provide your cat an ample, healthy feline menu from which to choose.
  2. Toys and Beds:  Cat toys and beds may be housed with your pet but cannot be returned to you.
  3. Hospitalization: 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 radioactive iodine therapy unit.  Once treated with the radioactive iodine, your cat cannot be released to you until his/her radioactivity levels are at, or below 0.50 mR/hr at 16.5 inches (1/3 meter), and hospitalized for a minimum of three days.  In the extremely unlikely event that a patient dies from another illness while being housed in the radioactive iodine therapy unit, we must hold their remains until radioactivity diminishes completely (eighty days).