(I-131) Radioiodine Therapy
Treatment for Hyperthyroid Cats 

Radioactive Iodine (I-131) Treatment for Hyperthyroid Cats

Hyperthyroidism is a very common condition in cats, especially as they age. Some cats may have other health issues as well, such as kidney disease. Consequently, we need recent health information prior to your cat’s treatment. These laboratory tests and X rays should be performed by your regular veterinarian and emailed to office@novacatclinic.com at least 1 week prior to their scheduled appointment. All appointments are tentative only until Dr. Brown reviews your cat’s medical history including the test results and X-rays.

Tests needed within 30 days prior to I-131 treatment:

  • Blood Tests Needed:

    • Complete Blood Cell Count (CBC)

    • Serum chemistry profile including electrolytes

    • Total T4 (not free T4) and TSH

  • Urinalysis including urine specific gravity

  • Chest X rays: Three views (right and left lateral and ventrodorsal views)

  • Blood Pressure: if possible; we will obtain blood pressure here when indicated.

  • Some cats may need additional tests based on their individual needs.

Note: your veterinarian should send a copy of your cat’s medical history along with the laboratory information and X rays.


Additional Information

If your cat is already receiving methimazole, we will need you to stop the methimazole and retest the Total T4 blood test 10 days later. We dose the I-131 based on the Total T4 levels, so we need an accurate un-medicated T4 for successful treatment.


In addition, all cats need to be off the methimazole for at least 10 days prior to the I-131 treatment.

Your cat will need to be quarantined in a separate room away from you and your family for 7 days after going home when no children under 18 or pregnant individuals are present in the home; the quarantine is 14 days when children or pregnant women are present.


Used litter needs to be flushed (or stored for 81 days) during the quarantine time. Each non-pregnant adult may spend up to 30 minutes per day in the quarantine room feeding, playing with, and giving TLC to their cat. During this time, you should be approximately arms- length distance from your cat rather than having it on your lap. If you are unable to quarantine your cat at home we can board your cat here for the necessary quarantine at an additional daily cost. We recommend home quarantine whenever possible as your cat will always prefer to be at home with you.

If you have concerns during your cat’s quarantine, please contact us immediately. Your cat cannot visit their regular veterinarian for 14 days after coming home from their I-131 treatment.

Your cat does, however, need to visit their regular veterinarian 2-4 weeks after treatment for their follow up examination and follow up laboratory tests.

Please have your veterinarian send us the results of the follow-up examination and laboratory tests as we want to make sure that your cat’s hyperthyroidism is cured and that they do well long term.

Please feel free to contact us at office@novacatclinic.com or call us at 703-525-1955 if you have any further questions. We look forward to helping you and your cat soon!