Radioactive Iodine I-131 is the only non-surgical definitive treatment for hyperthyroidism. When a cat or a person is hyperthyroid, the normal thyroid tissue has “gone to sleep.” The normal tissue has responded to feedback signals from the body that tell it to stop making the hormone. Radioiodine Therapy—I-131 for Cats Frequently Asked Questions What exactly is going to happen to my cat while he or she is at VIS? On the day of your pet’s arrival, a small injection of I-131. Whether your cat or your patient needs a thyroid scan for diagnosis, surgical evaluation, or pre-radioactive iodine treatment, we look forward to partnering with you to provide this service. If you have any questions regarding Angell’s I‑131 program, please email Dr. Duddy at i131@.