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Radioactive Iodine Therapy for Feline Hyperthyroidism
What is involved in the initial screening process?
The initial screening process includes: thorough physical exam, complete blood count and serum chemistry, total T4 value, and urinalysis. A urine protein:creatinine ratio is performed when overt proteinuria is identified. A microalbuminuria test is performed if urine protein is negative. 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 includes: survey thoracic radiographs, indirect blood pressure (usually via Doppler manometry) and a baseline electrocardiogram. Further cardiac evaluation (echocardiography) is not performed, unless there is suspicion or evidence of cardiac disease determined during this initial screening process.
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. Cats that are on Hills y/d food should be transitioned back to their regular diet prior to making an appointment for 131I evaluation. Cats should be off of y/d food for 4-6 weeks prior to 131I.
What is the next step after the initial screening?
Two diagnostic nuclear imaging studies are the next step in the process. 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 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 GFR should be above 2.25 mL/min/kg. If the GFR is below this number, medical therapy for hyperthyroidism may be recommended, although 131I treatment may still be an option in specific patients.
When is the 131I given?
Once the screening process is complete and the decision to pursue 131I treatment has been made, 131I is administered usually the day after the diagnostic scans. 131I is given as a one time, subcutaneous injection.
What is the scheduling for this?
Both diagnostic nuclear scans (thyroid scintigraphy and GFR) are currently performed on Wednesdays. The cats are required to spend 24 hours following the diagnostic nuclear scans in radiation isolation, even if they are not going to receive 131I the following day. Initial referral appointments should be scheduled on Mondays or Tuesdays. Cats may stay in the hospital until the diagnostic nuclear scans are performed, or they may go home between initial screening and initiation of scintigraphy. If cats are temporarily discharged, they must return to the hospital before 8 AM on the Wednesday of their diagnostic nuclear scans. Patients that fail to return after an order has been placed with the nuclear pharmacy will still be charged the radiopharmaceutical fee since the radionuclide cannot be returned.
How long are the cats in the hospital after 131I?
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 131I treatment. Clients may not visit the pet prior to discharge. Toys or blankets left with the cat in radiation isolation cannot be returned.
What should the client expect at the time of discharge?
Feline 131I 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 post-131I patient care instruction sheet.
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 131I cats at the time of discharge. Restrictions include handling of the cat and waste products with latex gloves, minimizing contact with the pet and special waste storage procedures.
What is the overall cost for 131I therapy?
The estimated costs for complete 131I work up and therapy range from $2,200 to $2,700 dollars.
Please contact the ISU-VMC at 515-294-4900 with any additional questions that you may have. Thank you.