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I-131 Therapy

3-13 002Has your cat experienced weight loss with a huge appetite? Has the weight loss been accompanied by increased meowing or irritability? If so, you may want to find out if he has hyperthyroidism.

With the advent of many improved medical treatments every day, Angell Animal Medical Center is constantly striving to find new ways to care for and treat companion animals. New treatment options and improved technology allow Angell to consistently remain one of the leaders in veterinary care. One particular specialty service offered on site is the I‑131 therapy program directed by Dr. Jean Duddy. I‑131 is a radioactive iodine treatment for felines battling hyperthyroidism, a disease which causes increased thyroid levels in the blood.

What is Feline Hyperthyroidism?

Feline hyperthyroidism is caused by thyroid masses, most of which are benign (called adenoma), and a few of which are malignant (called carcinoma). Fortunately, both conditions can be treated by radioiodine (I‑131).  For the adenoma (benign form), a small dose of I -131 is given.  The goal is to kill the tumor cells and attain normal thyroid hormone levels so the body can return to a normal, healthy weight. The overall success rate of this treatment is approximately 98%.  In the rare cases of adenocarcinoma (the malignant form) the goal is kill off all the thyroid tissue.  These kitties require a much higher dose and are in the hospital a longer period of time.  They also will require a thyroid supplement for life after the treatment.  This would all be discussed before the treatment.

What are some symptoms of Feline Hyperthyroidism?

  • Significant weight loss
  • Normal to increased appetite
  • Increased vocalization
  • Irritability and/or restlessness
  • Loss of normal hair coat

How Does I‑131 Work?

Thyroid scan of Three Jarm - lateral view with him in left lateral

A thyroid scan showing the thyroid tumor (white splotch at top).

The only gland in the body to store iodine in the cells is the thyroid gland.  The iodine we give is tagged with radiation so it is picked up by the blood stream, brought to the thyroid gland, and while it is in the gland waiting to be made into thyroid hormone, the radioactive iodine kills the tumor cells.  After the thyroid tumor cells are killed, the thyroid level falls and the normal thyroid tissue that is left starts to produce thyroid hormone again.

Will the Radiation Hurt the Normal Thyroid Tissue?

The normal tissue is relatively “protected” from the radiation because thyroid tumor cells continuously over-produce thyroid hormone.  During this time, the normal thyroid tissue turns off.  This automatic decrease of thyroid production (in normal thyroid tissue) due to high thyroid hormone levels in the blood is called “negative feedback”.  Tumor cells do not have this normal negative feedback mechanism.  They continue to produce thyroid hormone no matter how high the thyroid level in the blood is at anytime.  The tumor cells pick up the iodine tagged with radiation while the normal cells are not making thyroid hormone so they don’t pick up the radioactive iodine.


This kitty was unsedated and calm during his thyroid scan.

How Long Will the Treatment and Recovery Take?

Cats are admitted on Monday for any testing that needs to be done prior to the treatment.  The thyroid scan and treatment (a single injection of I‑131) are done on Wednesday.  The patients must remain hospitalized for 3 days post treatment so many owner’s pick up on Saturday following the treatment.  There are litter and contact restrictions due to radiation safety concerns for 17 days post treatment.  For owners who do not have the space or ability to deal with following these restrictions, these cats may remain at Angell for the entire restriction period. The thyroid level starts to fall in about 8-10 days so it can be a few weeks before cats stop losing weight and start to put weight back on. This is one of the reasons we recommend rechecking blood tests and weight 1 and 3 months post treatment.


13-year-old Ozzie emerges from his nap curious and relaxed after his treatment.

What Makes Angell’s I‑131 Program Unique?

  • We are the only thyroid treatment program in the Boston area that scans
    cats first. Some cats with high thyroid levels in the blood actually do not need to be treated because they do not have thyroid tumors.  The high thyroid level can be normal for the cat or due to another illness. The scan identifies these cats so that these cats are not given I‑131 unnecessarily.
  • Our program has a veterinarian on the premises 24 hours a day.
  • We are the only facility that handles cats with major medical issues (diabetes, asthma, etc.) that could complicate I‑131 treatment of hyperthyroidism.
  • Our license permits release after three days, but we also provide boarding for clients who do not want the responsibility of the radiation issues and handling concerns at home.

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

To schedule an appointment, or for further information please call the Internal Medicine service at 617-541-5186.

Pre-Treatment Reqs

Pre-Treatment Reqs

Pricing, Safety, Requirements before Treatment, and Discharge Instructions.

Download to view information including price ranges, requirements before treatment and after discharge, as well as safety precautions and details on how the procedure is done.

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