Feline I-131 Therapy

Radioactive Iodine (I-131) Therapy for Feline Hyperthyroidism at ISU

What is involved in the initial screening process?
The initial screening process performed by ISU Small Animal Internal Medicine includes: thorough physical exam, complete blood count and serum chemistry, total T4 value, and routine urinalysis. A urine protein:creatinine ratio is performed when proteinuria is identified. An SDMA test is performed if renal dysfunction is suspected. A free T4 may be run to confirm the diagnosis of hyperthyroidism if the total T4 is only marginally elevated or is within the high end of the normal range. The remainder of the screening process may include: survey thoracic radiographs, indirect blood pressure (usually via Doppler manometry) and a baseline electrocardiogram. Further cardiac evaluation (echocardiography) is not performed by the ISU Cardiology service, unless there is suspicion or evidence of cardiac disease determined during this initial screening process.

 Initial referral appointments for cats being considered for I-131 therapy should be scheduled on Mondays or Tuesdays to allow for diagnostic evaluation prior to I-131 treatment.

Can cats be on methimazole before referral?
Cats may be treated with methimazole before referral. It is required that cats be off methimazole a minimum of 7 days prior to referral (10-14 days preferred). Cats that are on Hills y/d food should be transitioned back to their regular diet prior to making an appointment for I-131 evaluation. Cats should be off of y/d food for a minimum of one week prior to I-131 therapy.

What is the next step after the initial screening?
Two diagnostic nuclear imaging studies are performed next. Thyroid scintigraphy is performed to assess for ectopic thyroid tissue, confirm the diagnosis of hyperthyroidism, and to evaluate the pattern of the uptake of the nuclear isotope within the thyroid tissue which may be consistent with benign adenomatous thyroid tissue. The second nuclear imaging study is performed to determine the renal glomerular filtration rate (GFR). GFR results are used to determine the functional status of the kidneys, and identify cats with subclinical renal disease. Ideally, the total GFR should be above 2.25 mL/min/kg. If the total GFR is below this number, medical therapy for hyperthyroidism may be recommended, although I-131 treatment may still be an option in specific patients.

Both diagnostic nuclear scans (thyroid scintigraphy and GFR) are currently performed on Wednesdays.  Cats are required to spend 24 hours following the diagnostic nuclear scans in radiation isolation, even if they are not going to receive I-131 the following day.  Clients who cancel diagnostic nuclear scans after an order has been placed with the nuclear pharmacy will still be charged the radiopharmaceutical fee because the radionuclides cannot be returned.

When is the I-131 given?
Once the screening process is complete and the decision to pursue I-313 treatment has been made, I-131 is administered the day following the diagnostic scans (Thursday). I-131 is given as a one time, subcutaneous injection.

How long are the cats in the hospital after I-131 is administered?
The average stay is 4-5 days post-injection. This means that most cats will be discharged from the hospital the following Monday or Tuesday after I-131 treatment. Clients may not visit the pet prior to discharge, but will be updated daily on the health status of their pet by ISU Small Animal Internal Medicine. Toys or blankets cannot be accepted in radiation isolation.

What should the client expect at the time of discharge?
A detailed post-treatment instruction handout is signed by the pet owner prior to patient release from radiation isolation.  Feline I-131 patients should be transported home in a cat carrier and handled with latex gloves. A waste receptacle will be provided at the time of discharge along with a detailed post-I-131 patient care instruction sheet.  Cats inside cat carriers should be placed in the back floor board or back seat of the car and should not be seated on any pet owner’s lap.

What restrictions are there once the cat has gone home?
The client must sign and retain the provided instruction sheet for home care of the I-131 treated cat at the time of discharge. Restrictions include minimizing contact with the pet, especially for the first two weeks, handling of the cat and waste products with latex gloves for 6 weeks, and special waste storage procedures for the first two weeks.

What is the total cost for I-131 therapy?
The estimated costs for complete I-131 work up and therapy range from $2,200 to $2,700 dollars.

Recheck evaluations with the client’s referring DVM for evaluation of T4 levels at 1 and 3 months post-discharge are recommended.

For more information, please contact the ISU-VMC at 515-294-4900 with any additional questions that you may have. Thank you.