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The feline radioactive iodine clinic has been set up by Vicky Maund (Lead Radioactive Iodine Nurse and Internal Medicine Nurse) and Stephanie Lalor (Internal Medicine and Feline Specialist). Vicky Maund is in charge of the daily running of the clinic.
What is feline hyperthyroidism and why treat with 131I
Feline hyperthyroidism is a common endocrine disorder. Clinical symptoms include polyphagia, weight loss, muscle wastage, hyperexcitability, nervousness, behavioural changes, tremors, and heat intolerance. Initially diagnosis of the condition is made with a blood test, and potentially investigated further to detect any concurrent underlying conditions such as renal failure, hypertension and/or cardiac disease as these can impact on the cat’s wellbeing. Treatment for the condition can be undertaken with oral medication for the remaining life expectancy of the cat, a specialised diet, surgical intervention to remove the thyroid gland(s), or radioactive iodine therapy rendering the benign adenomatous hyperplasic tissue inactive.
Radioactive iodine therapy can be considered the gold standard therapy with a success rate of 95%. It is also ideal for patients that are unsuitable for medical treatment or surgical thyroidectomy. Most cats have a normal thyroid hormone level after one treatment of radioactive iodine. The thyroid level however can go too low which may necessitate your cat requiring oral thyroid hormone supplementation. The radioactivity is very local and there is no hair loss or other side effects that are associated with radiotherapy. It is well documented that 20% to 25% of hyperthyroid cats have multiple areas of hyperfunctional thyroid tissue, which is destroyed with radioiodine treatment regardless of position. All treatment regimes for hyperthyroidism can potentially unmask kidney disease; therefore, it is important to monitor this closely.
Radioactive iodine therapy is extremely safe and effective but does require hospitalisation of the patient for up to four weeks. The procedure does not necessarily require a general anaesthetic / sedation.
Blood and urine samples should be carried out before any treatment is administered checking for any underlying medical conditions. If kidney disease is diagnosed treatment should initially be oral thyroid medication to make sure that the kidney disease does not significantly worsen with successful treatment of the hyperthyroidism. The cat should have antithyroid medication discontinued for 10 – 14 days to allow for a washing out period.
Due to the radioactive nature of the treatment strict health and safety regulations must be adhered to when nursing these patients. The client must be well informed of this in case of potential hazards within the home setting. Time spent with the cat must be limited and it is recommended that the cat must not come into prolonged contact with children or pregnant women. Disposal of all excretions must be carried out in accordance with the guidelines set out by Willows so as to not only ensure owner health and safety but also that of others. This may be a difficult time for owner’s due to the previous period of isolation and in a home setting potentially the owner may disregard the safety guidelines but the importance of restricted contact must be reinforced.
Owners must still be advised that although most cats are cured TT4, renal and blood pressure monitoring should continue on a regular basis to identify potential future relapse. Thyroid hormone concentration should return within the normal reference range parameters within two weeks and in 95% of cases within three months.
Protocol for treatment of cats
Criteria for patient selection prior to 131I
- Cats must be referred by a Veterinary surgeon who has clinically documented hyperthyroidism
- Cats must be in otherwise good health - no congestive heart failure, significant renal failure (not ideal candidates), or other serious health problems
- Recommend having started medical treatment while waiting for treatment
- Some cats will not be able to be controlled so will need to be treated without any prior medical management - idiosyncratic reactions or impossible to control despite high doses
- Cats belonging to owners who exhibit anxiety about radioactive material will not be accepted for treatment
- Owners must agree to be separated from the cat for a minimum of two weeks during therapy confinement
- Owners must sign a consent form confirming that post-therapeutic procedures will be followed if the cat leaves the unit after 15 days.
- Owners with minor children or pregnant women living in the home will be carefully evaluated before a cat is selected for treatment. If a decision is made to treat, detailed information will be given about avoiding contact between the treated animal and these individuals
- A deposit will be required prior to the appointment.
The cat does not experience any adverse side effects from the radioactive iodine. Because the delivery of radiation is targeted to the overactive thyroid gland, the cat does not experience any radiation side effects at the normal therapy doses used to treat hyperthyroidism. The medicine is give as an injection, usually 2 days following admittance to the clinic as blood work needs to be done prior to the injection. Following the treatment, the cat will be hospitalised for 15 - 23 days to allow most of the radioactive medicine to leave the thyroid gland or decay prior to discharge from the clinic. This is different from the situation in human nuclear medicine as most people treated with radioactive iodine for hyperthyroidism are discharged the same day they are treated.
You cannot visit your pet during treatment, nor can pets be removed from the ward until officially released. You cannot terminate treatment or arrange for early release once therapy has begun. Pets may not be boarded/hospitalised elsewhere until they meet the requirements for release. You may leave a small bed or personnel item such as a jumper or t-shirt for comfort but this will not be returned to you.
After being released from treatment, your cat will still possess a very low level of radioactivity, being voided out primarily via urine and faeces. You do not need to totally isolate your cat from people/pets, but you must follow safety precautions until the date listed is reached. Due to the natural decay of radioactivity and continual loss of radioiodine through the urine and stool, your cat will contain no detectable level of radioactivity soon after that date.
During hospitalisation, cats are housed in individual enclosures in an isolation room in the clinic.
Bedding is changed regularly and fresh food and water are available at all times. Within one to three months after therapy, the majority of hyperthyroid cats become normal (euthyroid), up to 30% will become hypothyroid (too little thyroid hormone in the blood) and may require oral thyroid hormone replacement therapy, and 5-7% remain somewhat hyperthyroid. Cats with persistent hyperthyroidism can be retreated after their initial therapy but will need additional investigations to try and rule out a functional thyroid carcinoma.
To be candidates for radioactive iodine therapy, all cats have screening laboratory work (blood work, thyroid hormone analysis, and urinalysis) performed by the referring veterinary surgeon within one-month prior to the anticipated treatment date, but we will repeat these bloods to ensure an accurate dose of 131I is administered. We must have copies of this lab work before your cat comes for treatment. Aggressive cats and cats with chronic renal (kidney) failure and/or advanced heart disease are not candidates for radioactive iodine therapy.
Follow up appointment will be required 1 month and 5-6month after discharge. This is for repeat blood work, urinalysis and blood pressure assessment.