Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066





(Photo Credit: Advanced Veterinary Medical Imaging

This method of therapy is generally considered the safest and most effective method of treatment for feline hyperthyroidism. The procedure involves first a nuclear medicine scan in which the cat receives an injection of the radioactive compound pertechnetate. The resulting scan shows the location and size of the cat’s thyroid glands and confirms the disease. The scan also indicates whether one or both glands are involved and provides information needed to calculate the therapeutic dose of iodine 131 that will be used in treatment. An addtional benefit of the scan is that it can identify the 3-5% of cats who have a malignant tumor and detects areas of tumor spread.

For information about malignant thyroid tumors click here.

After the pertechnetate scan is complete, malignancy has been ruled out, and hyperthyroidism has been confirmed, iodine 131, a radioactive isotope of iodine, can be used to destroy the abnormal thyroid tissue. Normally, iodine is joined to the amino acid tyrosine in the thyroid gland to create T4. Iodine 131 is carried directly to the thyroid gland as though it were regular iodine. Iodine 131, being radioactive, emits high speed electrons which kill the surrounding abnormal thyroid tissue. Because these electrons penetrate only fractions of an inch, only the thyroid gland experiences the radiation and the rest of the body is spared.

This treatment need not be repeated and no additional therapy is required; however, while humans receiving similar treatment are promptly allowed to go home post-treatment, the process is different with cats. The Nuclear Regulatory Commission requires cats to remain hospitalized in a special isolation ward until the iodine 131 is at sufficiently low levels. This amounts to about three or four days of hospitalization depending on how quickly radiation levels drop. After the cat comes home, some restrictions must be imposed for another week and a half or so: the cat must use special flushable litter, the cat is not allowed outside, the cat's daily direct contact time with the owner is limited, children and pregnant women may not have contact with the cat during this time etc.

Blood work is monitored following treatment to ensure return to normal thyroid status. Occasionally, a single course of radiotherapy is inadequate and a second course is needed (see below).



  • Treatment is a one time event (only 2 to 4% of cats require a second treatment) and no on-going therapy is required.
  • The disease is not simply managed but is actually cured!
  • No anesthesia is required, indeed, treatment amounts to an injection followed by 3 to 7 days of boarding, very non-stressful for older cats with potential heart disease.
  • If a cat is one of the unlucky 3 to 5% for which the thyroid tumor is malignant, the initial pertechnetate scan will indicate this right away.



  • Owner and pet are separated during the quarantine.
  • Some facilities require the cat to be confined indoors or have limited contact with owners for a period of time after discharge from the radiofacility. Children and pregnant women can have no contact with the cat for a week or two after therapy. If this is too inconvenient to work out at home, the cat may be boarded until this period has passed.
  • Facilities with capability of performing radiotherapy may not be conveniently located.
  • This is a relatively expensive therapy. (In Southern California $900 is typical).
  • Special flushable cat litter is required for 1-2 weeks after therapy.
  • Some follow-up blood testing is generally recommended after treatment (typically one and three months after therapy).
  • There is a chance (less than 5%) that the cat will become HYPOthyroid after treatment, requiring daily oral thyroid hormone supplementation.
  • Radiotherapy may not be a good idea for a cat with poor kidney function.

If kidney function is not thoroughly investigated prior to this therapy, latent kidney failure may be unmasked irreversibly by this therapy. This can be avoided simply by screening potential candidates for kidney failure prior to recommending radiotherapy. Those who have possible kidney insufficiency should be treated with medication to bring the thyroid levels under control. If kidney function begins to show deterioration on this therapy, medication is discontinued and one must reevaluate the need for treating thyroid disease. If kidney function remains stable on treatment with anti-thyroid medications, then a more permanent therapy (such as radiotherapy) can proceed.



For a list of facilities known to us in the United States and Canada that can offer Radiotherapy Treatment for your cat, click here.

Page last updated: 4/30/08
Page last reviewed: 8/13/2014