A single dose of radioactive iodine (I131) is given by subcutaneous injection. The iodine is concentrated in the thyroid gland where it emits beta-radiation, killing the surrounding hyperfunctioning cells. Parathyroid gland function is unaffected. The iodine that is not concentrated in the thyroid gland is very rapidly eliminated in the urine, saliva and sweat in the first couple of days following the injection. The remainder is very slowly eliminated due to thyroid hormone turnover (with the I131 incorporated into the hormone) and due to decay of the isotope (the half-life of I131 is eight days). Cats must stay with us in an isolation facility for at least 10 days after the injection until most of the radioactivity has been eliminated. A nurse feeds the cats and cleans their cages every morning. Lighting and daytime stimulation such as radio or cat television is altered throughout the day.

The current cost of treatment is £1425.00 inc. VAT. This cost is essentially a ‘package deal’ covering pre-treatment investigations, the treatment itself and hospitalisation for 10 days. This does not include the cost of treatment for any concurrent medical problems or any unexpected diagnostic tests.

A single radioactive iodine injection is successful in about 95% of cats treated.

The thyroid hormone concentration is generally within normal limits one month after the radioactive iodine injection. In some cases it can take up to six months for the thyroid hormone concentration to return to normal following treatment.

1. Cats need to be confirmed as hyperthyroid on the basis of a total thyroxine (tT4) concentration above the laboratory reference range. We will treat cats with high - normal total Thyroxine (tT4) measurements where hyperthyroidism has been confirmed by free T4 assuming compatible clinical signs are present. If a case is accepted for referral on the basis of a free T4 measurement, we will require repeat free T4 (by equilibrium dialysis) and total T4 measurements approximately two weeks prior to the referral appointment – we will advise you of this accordingly.

2. Assessment of renal function. Renal function is profoundly influenced by thyroid status – excessive thyroid hormone concentration leads to increased cardiac output resulting in an increase in glomerular filtration rate and consequently a decrease in circulating creatinine concentration. Normalisation of total T4 concentration induces a decline in GFR and can therefore unmask underlying renal abnormalities. Studies show that a decrease in GFR will occur within 4 weeks of radioactive iodine treatment with little decline after this point, but for a proportion of cats this can result in the development of azotaemia (creatinine greater than 140μmol/L) which can take up to six months to manifest.

• For newly diagnosed cats that are non-azotaemic (creatinine less than 140μmol/L) and have a urine specific gravity of greater than 1.040 the magnitude of this decrease in GFR has been shown to be no more than one IRIS stage, which at worst could result in the unmasking of IRIS stage II renal disease. As the development of mild renal disease is not associated with a decreased survival time unless associated with hypothyroidism, we no longer require a period of stabilisation as a therapeutic trial to assess the effect of treatment on renal function. If there is a waiting list for treatment we will advise you to start either medication or Hills y/d diet: in this case blood tests will be required 4-6 weeks prior to the appointment with us.
• Cats that are azotaemic at diagnosis or cats whose owners feel that any development of azotaemia should be avoided where possible do still benefit from a therapeutic trial of anti-thyroid medication to assess the renal response to reversible treatment before radioactive iodine treatment is considered.
• Euthyroid cats on medication or y/d diet that are non-azotaemic or have stable IRIS stage II renal disease (creatinine less than 250μmol/L) are suitable candidates for radioactive iodine treatment.
• If the reason for referral for radioactive iodine treatment is that the cat cannot tolerate anti thyroid medication or the owner is not able to medicate the cat, radioactive iodine treatment is still possible assuming the cat is non-azotaemic prior to referral.
• In order to assess renal function we require testing of urine specific gravity and serum urea, creatinine and phosphate. If there is a long delay between the blood tests to assess renal function and the appointment with us, further blood tests should be performed 4-6 weeks prior to the appointment to ensure nothing significant has changed.

3. Cats need to be vaccinated for flu and enteritis within the last year.

All cats treated with radioactive iodine will stay for a minimum of 10 days following the injection. They can then be discharged assuming a number of conditions at home can be met for a further 18 days, after which no further precautions are required. These conditions are that:

• The cat will be confined indoors, without access to occupied bedrooms and will use a litter-box.
• The owners can restrict the time spent cuddling the cat or holding the cat on their lap.
• The owners are able to ensure that any children in the household will remain at a safe distance from the cat and its litter-box.
• There is no-one in the house that is pregnant, or trying to become pregnant.
• There is a secure outside storage area (garage, shed) where soiled litter can be stored for one month before being put out for collection. If this is not possible, special litter can be purchased that can be flushed down the toilet, but the household plumbing must be in good working order to do this.

If cats are discharged after 10 days, their owners will have to sign a document stating that these conditions will be met. If these conditions cannot be met at home, cats may board with us for the additional 18 days at a cost of £30.58 per day. The level of radioactivity emanating from the cats 10 days after treatment is relatively low, but this is considerably higher than background and will continue to be so for several weeks. The risks associated with this radiation level are small provided that sensible precautions are taken.

1. Thyroid carcinoma. Although thyroid carcinoma can be treated with radioactive iodine, the doses of radioactivity required are greater than those that we have authorisation to hold, so we are unable currently to offer treatment to these cats. There is increasing evidence that hyperthyroidism is a spectrum of disease and that transformation to carcinoma may occur over time: therefore cats that have been treated medically for a long time and in whom increasing doses of anti-thyroid medication are required to maintain euthyroidism may be at risk of carcinoma development.

2. Naughty cats! We appreciate that hyperthyroid cats can be feisty and it is not a particular problem if, for example, a cat is difficult to collect blood samples from. The cats that we cannot accept for treatment are ones that will not allow their cages to be cleaned without attacking the Nurses, which increases the risk of contaminating them with radioactive urine.

3. Cats with significant concurrent medical problems. If cats become ill after they are injected, we cannot attend to them without being exposed to high levels of radioactivity. Therefore we cannot take cats that are known to have other serious concurrent problems. Medications can be put in the cat’s food, but the cats cannot be given tablets directly.

4. Unsuitable owners! Some cats would be fine to treat with radioactive iodine, but their owners will not be parted from them for required time. We are unable to compromise on the amount of time that we keep the cats here in the Hospital. It is not possible for owners to visit while their cats are with us.