Feline Hyperthyroidism

Radioactive Iodine (I-131) Treatment

Feline hyperthyroidism, is a common disorder in middle-aged and older cats. It occurs in about 10 percent of cats over 10 years of age. Hyperthyroidism is a disease caused by an overactive thyroid gland that secretes excess thyroid hormone. Cats typically have two thyroid glands, one gland on each side of the neck. One or both glands may be affected. The excess thyroid hormone causes an overactive metabolism that stresses the heart, digestive tract, and many other organ systems.

If your cat is diagnosed with hyperthyroidism, they should receive some form of treatment to control the symptoms. Many cats that are diagnosed early can be treated successfully. When hyperthyroidism goes untreated, clinical signs will progress leading to marked weight loss and serious complications due to damage to the cat’s heart, kidneys, and other organ systems.


If you see any of the following behaviors or problems in your cat, it possible that your cat has hyperthyroidism:

  • weight loss despite a normal or increased appetite
  • increased urination, more urine in the litter box
  • increased drinking or thirst
  • defecation outside of the litter box
  • increased vocalization
  • restlessness, increased activity
  • vomiting
  • diarrhea
  • rarely, lethargy and a lack of appetite
  • poor hair coat, unkempt fur


Twice yearly examinations of your cat may allow early detection of hyperthyroidism, as well as other age related diseases. During the physical examination, your veterinarian may discover increased heart and respiratory rates, hypertension, a palpable thyroid gland, and loss of muscle mass. Routine screening of laboratory tests and blood pressure may detect abnormalities before clinical signs (bulleted list to left) are advanced. Blood testing can reveal elevation of thyroid hormones to establish a diagnosis of hyperthyroidism. occasionally, additional diagnostics may be required to confirm the diagnosis. Because hyperthyroidism can occur along with other medical conditions, and it affects other organs, a comprehensive screening of your cat’s heart, kidneys, and other organ systems is imperative.


If your cat is diagnosed with hyperthyroidism, we will discuss and recommend various treatment options. Three common treatments for feline hyperthyroidism are available and each has advantages and disadvantages. The choice can depend on factors such as the cat’s age, other disease conditions, treatment cost, and our professional recommendation.

Radioiodine therapy – commonly called I-131. This treatment consists of administering a small dose of radioactive iodine which only overactive thyroid tissue will absorb. The radiation destroys the abnormal cells while the normal thyroid tissue continues to function. Even though this radiation exposure carries minimal risks for you and your cat, special facilities are required for treatment, and specific isolation protocols need to be followed after discharge. The advantages of I-131 treatment are that it can be curative and there is no anesthesia, surgery, or risk of drug reaction. We are one of a few facilities that provide this therapy as specialized and licensed treatment center.

Thyroidectomy – a surgical technique which removes all or part of the thyroid gland. The advantage of surgery is that it can be curative and eliminate the need for life-long medication. The disadvantages of surgery are that your cat requires general anesthesia and not all cats are good surgical candidates. Additionally, varying complications of surgery may occur including damage to nerves and blood vessels of the neck, damage to the parathyroid gland function, and recurrence of hyperthyroidism as unrecognized tissue can remain even with the best surgical techniques.

Medical therapy – anti-thyroid medications will control the disease and block the excess production of the thyroid hormone; however because this medication does not cure the disease, your cat must take it for its entire life. Your cat may also receive the drug as a short-term measure, prior to surgery or anesthesia, or if radioiodine therapy is not available right away.

Advantages of medical -- therapy are a low initial cost, readily available treatment, and no hospitalization. Disadvantages include the need for medication, potential for adverse drug effects, and long-term costs of treatment.

Radioactive iodine (I-131) is the preferred method of treating feline hyperthyroidism.

As the primary care provider, you can perform ALL pre and post treatment testing. We are available to consult with you at any time before, during, and after I-131 treatment.

Advantages include:

  • 98%+ success rate.
  • No lifetime medication regime.
  • No surgery or anesthesia.
  • No effect on normal thyroid tissue or the nearby parathyroid tissue.
  • The total cost of I-131 is less than other treatment methods over the lifetime of the cat.


1. Laboratory evaluation - completed 30 days prior to treatment and submitted to SWFVS for review prior to scheduling.

  • CBC, complete chemistry profile, urinalysis w/ reflex UPC, total T4 by an outside lab
  • Two - three view thoracic radiographs
  • NT proBNP is optional, but recommended for cats with a heart murmur, tachycardia, gallop rhythm, and/or radiographic cardiomegaly.
  • Please provide all previous T4 values, and all pertinent medical records including history of azotemia, biopsies, neoplasia, or other illnesses.

2. Methimazole trial:
  • Can be performed at on any patient, but may be recommended for some patients after review of lab results. For patients on methimazole, assessment of renal values and USG is recommended once patient is euthyroid.
  • Cats MUST be off methimazole for 2 weeks prior to treatment. A T4 should be performed 10-14 days after cessation.

3. Rabies vaccination must be current.


1. Patients are hospitalized in the nuclear medicine ward for 3 nights.

2. Treatment package includes:

  • Review of all pertinent case records and radiographs.
  • Physical examination and assessment of Doppler blood pressure.
  • Radioisotope (I-131) and appropriate radiation monitoring.
  • Daily care and feeding and hospitalization in the nuclear medicine ward.
  • Three month and/or as needed follow-up consultations between SWFVS and the primary care veterinarian.

3. Does not include:
  • Diagnostic testing performed at your clinic prior to admission to SWFVS.
  • Additional testing such as an echocardiogram and/or abdominal ultrasound, if deemed necessary.
  • Emergency medical tests, procedures, or medications that may unexpectedly arise during hospitalization.

Any post treatment lab work performed at your clinic, typically required at 4 weeks and 3 months post-treatment.

4. Daily patient updates are provided to the client.

5. Clients are unable visit during treatment and cannot terminate once beginning due to radiation safety guidelines.

Post treatment:

1. Patients are released in accordance with strict Federal regulations. Patients will excrete small amounts of radioiodine post discharge. Clients are instructed on the safe handling procedures for two weeks post-release. If clients are unable to comply with these precautions, medical boarding is available.

2. Post treatment lab work is recommended at 4 weeks and 3 months. SWFVS can consult with you regarding follow up.

3. Possible complications of I-131 therapy include:

  • Underlying renal dysfunction may become unmasked following treatment.
  • Hypothyroidism, which in rare cases may require exogenous thyroid supplementation.
  • Transient sore throat or dysphagia. A permanent voice change is possible.

SWFVS is excited to partner with you to offer this service to your clients. They most certainly will be grateful for the care and concern you have shown in finding a cure for their pet’s hyperthyroidism.