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Radioiodine Therapy (I-131)

Considered the best option to cure hyperthyroidism today, radioactive iodine (131-I) is a minimally invasive alternative to thyroid gland surgery or daily medication. Over 95% of cats will be cured with a single injection. Radioactive iodine offers a simple, effective, and safe form of therapy for cats suffering from hyperthyroidism, with limited risks and disadvantages.

Iodine is used naturally by the thyroid gland, but by no other gland or organ in the body. Therefore, when radioactive iodine is injected into the body, it goes right to the thyroid gland and is absorbed by the hyperactive thyroid cells. The more hyperactive they are, the more radioiodine is accumulated, eventually killing the hyperactive cells. However, normal cells typically return to normal function once the radioiodine is excreted from the cat’s system.

The radioiodine does not travel elsewhere in the body and neighboring structures are not affected. The radioiodine procedure avoids daily medicine doses and side effects of medications, in addition to avoiding the risks and recovery time of surgical intervention.

Cats admitted for radio iodine therapy will spend 3-4 days at LIVS in comfortable accommodations until it is safe for them to return home.


Advantages of Radioiodine (I-131)

  • Daily pilling eliminated
  • More cost effective than surgery or daily medication
  • No anesthesia required
  • Normal thyroid tissue is not affected
  • Destroys thyroid tumors
  • Thyroid function is returned to normal within a few weeks

Disadvantages of Radioiodine Therapy

The major disadvantage associated with radioiodine therapy involves the required
quarantine in the hospital after treatment for a specified period (3-5 days), with visitation not permitted.

Which patients are good candidates for therapy?

Ideally, patients should be free of significant/advanced cardiovascular, renal,
gastrointestinal, or endocrine disease, and not aggressive in nature. Most patients presenting for evaluation are middle to older-aged cats, often suffering with comorbid conditions, and should be assessed for stability prior to definitive treatment.


What preparation is required prior to therapy?

Prior to therapy a baseline complete blood count (CBC), serum chemistry profile, urinalysis, and total T4 should be submitted. If the patient is currently receiving methimazole, T4 submission should be conducted between 7 to 14 days after discontinuation of therapy to assess disease severity. If a free T4 is submitted, it should always be accompanied by a total T4, as the diagnosis of hyperthyroidism is never established solely on the finding of an elevated free T4. Ancillary diagnostic investigation should be performed if the cat has evidence of clinically significant physical abnormalities (thoracic radiographs, echocardiography, abdominal ultrasonography). The institution of medical management (methimazole trial) to assess patient stability upon return to euthyroid state should be considered prior to radioiodine therapy if concurrent renal disease is suspected to ensure stability of renal function.

Many specialists and consultants are necessary to ensure the safety and quality of the radioiodine/radiation program.

Radioiodine Team


Radioiodine Therapy I-131


Medical/Radiation Oncology Consultants

Lee O’Donnell (Radiation therapist)

Kenneth Satchwill, MS (Dosimetrist)

Radiation Safety Officers