Radioactive Iodine Treatment at the Purdue Veterinary Hospital

Feline hyperthyroidism is a very common disorder of older cats usually caused by a benign tumor in the thyroid gland. Hyperthyroidism is caused by excessive production of thyroid hormone by the thyroid gland, which can lead to signs such as behavioral changes, hyperactivity, weight loss, high blood pressure, and heart disease in aging cats. One of the most effective treatments for feline hyperthyroidism is radioactive iodine (Iodine¹³¹). Once administered, the radioactive iodine concentrates within the thyroid gland and the emitted radiation destroys functional thyroid tissue without causing radiation damage to the normal parts of the thyroid gland or other tissues in the body. The advantage of this treatment is that it is safe, no anesthesia is required, and it is effective for the treatment of ectopic thyroid tissue or malignant thyroid tumors (at higher doses). Because of the long half-life of radioactive iodine, cats need to be kept isolated for a few days after treatment. The typical hospitalization period for evaluation and treatment in our facility is 5-7 days, although cats from households where there are children or pregnant women, or cats treated for malignant thyroid tumors may need to stay a few days longer.

In our program, the typical dose administered is 3-4 millicurie, with reduced doses considered in cats that have chronic kidney disease or very mild hyperthyroidism. Side effects of the treatment may include a slight worsening of kidney function and some risk of reduced thyroid function (hypothyroidism). 

1. In many cases, we recommend trial treatment with methimazole (tapazole, felimazole) prior to radioactive iodine treatment to make sure that treatment of hyperthyroidism does not worsen your cat’s kidney function. Your veterinarian will work with you to make sure that your cat is a good candidate for radioactive iodine treatment. There are cases where administration of methimazole is not possible, and this does not exclude your cat from iodine therapy. In these cases, we just recommend that your veterinarian consult with us for advice as to whether your cat is a good candidate for treatment.
2. A complete blood count, serum chemistry profile, urinalysis, total thyroxine (T4) concentration, and chest x-rays should be performed by your veterinarian during the 2 weeks prior to your appointment.
3. Your cat should be up to date on vaccines prior to your appointment (at minimum Rabies vaccine, and preferably (but at veterinarian’s discretion) feline distemper, feline herpes, and feline calicivirus.
4. Any treatment being used for your cat’s hyperthyroidism should be discontinued two weeks prior to your appointment (e.g. methimazole or the y/d diet).Your cat should fasted starting at midnight before their appointment at Purdue but please do not limit water.
5. Please bring enough of your cat’s usual food with them to last the duration of their stay (approximately 7 days), so that they can stay on the same diet they eat at home. You can also bring toys, blankets and a bed but those will not be able to be returned to you because of the risk of radioactive contamination.

What to expect when you bring your hyperthyroid cat for treatment to the Purdue University Hospital

1. A veterinary student and a PVH clinician in small animal internal medicine will take a history, perform a physical examination including a blood pressure, and review your cat’s test results. We will also collect a small blood sample to measure the pre-treatment total T4 concentration.
2. The labwork performed by your veterinarian is not likely to be repeated unless you report new concerns about your cat which have developed before your appointment with Purdue, or unless we identify any concerning abnormalities on our physical examination. Any additional tests we felt were necessary will be discussed with you prior to submission.
3. If we determine that your cat is a good candidate for treatment we will admit them to the hospital and order the correct dose of radioactive iodine for your cat. We will also make sure that they are comfortable and willing to eat in the hospital.
4. The following day we will move the cat to our isolation facility and administer the radioactive iodine treatment by subcutaneous injection.
5. In the isolation facility, your cat will have a private cage with a bed, litter box and ad-libitum food and water. Our staff will check on your cat at least twice a day and a veterinarian will check on your cat at least once a day.
6. When your cat’s background radioactivity has dropped to a safe level, we will collect another small blood sample to measure the post-treatment T4 concentration to make sure your cat has responded well to treatment.
7. Prior to taking your cat home, you will be asked to sign a release form, which explains how to take care of your cat at home. We will provide instructions on how long your cat needs to be kept inside, how long you need to keep your cat at a safe distance after release, and how to dispose of the cat litter (add link) .
8. After release from the hospital we recommend that the total T4, kidney function, and blood pressure are rechecked by your referring veterinarian, at 1, 3, 6 and 12 months after treatment.

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