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I-131 therapy

Feline Hyperthyroidism & I-131


I-131 therapy is an effective treatment for hyperthyroidism in cats. In approximately 95% of cases, the disease is cured after one injection, which is given under the skin. Iodine is an element required for normal health and is primarily used by the thyroid gland to produce thyroid hormones.

When iodine that has been made radioactive is injected into the body, it accumulates in the thyroid gland. The rest of the I-131 is excreted in the urine and feces. When the I-131 is taken up by the thyroid gland, radiation is released which kills the thyroid tumor cells.


Contact Information

Dr. Ariel Mosenco, DVM, DACVIM
Email: amosenco@vet.upenn.edu
Phone: 215-898-5439

Victoria Anders, CVT, VTS (SAIM)
I-131 Nurse
Email: victorij@vet.upenn.edu

Referrals & Consultations: 215-898-4218

Alli and Mosenco tp

Radioactive Cats!

Church wasn’t acting like herself. The 13-year-old gray cat, named for her doppelgänger in the film Pet Sematary, was drinking more water than usual and was abnormally excitable. Her devoted owners, Lauren Catullo and Rianna Taylor, noticed the changes immediately. Thankfully, Church’s yearly veterinary exam was right around the corner.

How is I-131 administered? Are there any restrictions after the treatment?

I-131 treatment for hyperthyroidism in catsThe I-131 treatment is given as one relatively painless subcutaneous injection and does not require anesthesia. Radiation levels are monitored until they are low enough for the patient to go home, 4-5 days after admission. The side effects of the injection are minimal.

I-131 works by radioactive destruction of the overactive thyroid gland, and thus, reduction of thyroid hormone. The radioactivity is excreted in the urine, feces, and saliva of the treated cats and they must be quarantined until the majority of the radioactivity is eliminated. Owners must be willing to keep the patient indoors, use clumping or flushable litter, and restrict human contact (at home) for two weeks.

Side effects of treatment are hypothyroidism in approximately 5% of cats. This may require supplementation with thyroid hormone. Approximately 5% of cats may remain hyperthyroid after treatment and may require another injection of I-131. No other medication is required after successful treatment.

What diagnostics need to be performed prior to referral?

Evaluation for these patients should include a complete blood count, complete chemistry screen, total T4, urinalysis, urine culture if indicated, and thoracic radiographs within one month prior to examination at PennVet. We recommend that cats admitted into the program have normal renal panels after control of hyperthyroidism. Animals receiving methimazole (Tapazole) can be treated with I-131; however, the methimazole must be discontinued three days before treatment.

Candidates not currently being treated by Ryan Hospital should be evaluated by a Ryan Internal Medicine clinician. During the initial appointment, the cat will be examined and prior test results will be reviewed. If the information is sufficient and non-thyroidal disease is not evident, the cat will be scheduled for I-131 treatment. If any other testing is required, the clinician will discuss this with the owner. Other tests usually can be completed on the appointment day.

What is included in the treatment price?

I-131 patient in hospitalPlease contact us for the most current price. Our pricing includes:

  • I-131 injection
  • Hospitalization
  • Radioactive waste handling
  • Monitoring
  • Examination fee for the first recheck after I-131 treatment

The initial internal medicine consultation prior to treatment, additional tests deemed necessary, and thyroid hormone monitoring post-treatment are not included. By carefully screening our patients, we anticipate that emergency medical care will be rare. However, in the event that the patient requires additional medical attention during the I-131 treatment, additional costs will be incurred. Any such treatment will be discussed with the owner first, except in an extreme emergency.

How long will my patient be in the hospital?

The patient will be admitted to the hospital on Monday and will receive the I-131 injection that day. Patients are monitored daily for radioactive output and discharged when the output is at a safe level. This generally occurs in time for discharge on Thursday afternoon of the same week.

Special procedures must be followed by the owner for the following two weeks; these are outlined in a separate handout provided upon discharge.

medicine team 2016

Owners will not be allowed to visit their pets during treatment; however, during their stay, the cats will be regularly monitored throughout the day and night, although handling will be minimal.

Owners may bring food and even bedding from home (but anything that has been in the cages with the cats cannot be returned).

Patients are fed as closely as possible to their usual diet and feeding schedule at home, and litter boxes will be cleaned regularly. The cages are comfortable, with sleeping ledges and separate litter box areas.