Small Animal Referral Hospital Radioactive iodine treatment for Feline Hyperthyroidism: Information for owners

Hyperthyroidism is the most common hormonal disease seen in older cats. The disease is caused by excessive thyroid hormone production due to a tumour in the thyroid gland(s). In 97- 99% of cases the tumour is benign (an adenoma or hyperplasia) and rarely a malignant tumour (carcinoma). The signs seen in hyperthyroid cats are due to the high levels of thyroid hormone. Common signs include weight loss, excessive hunger and thirst, vomiting, diarrhoea, agitation and vocalisation. Rarely cats have a form of the disease called ‘apathetic hyperthyroidism’ where they may be lethargic and inappetant, as opposed to showing the more typical signs. In most cats the enlarged thyroid gland(s) can be felt within the neck. However, between 5 to 20% of cats may also have ectopic tissue within the chest.

You need to discuss this with your own local vet, who will then be able to refer you to the Feline Centre; as we are a referral hospital we cannot accept requests directly from owners. Our service routinely starts with a telemedicine consultation (by Microsoft TEAMS or telephone call), to discuss all the previous medical history. During this teleconsultation, we would organise an assessment appointment (usually over two days), to determine if they are suitable to receive radioactive iodine.

As most cats with hyperthyroidism are senior or geriatric, it is very common for them to have concurrent disease, which may not be visible from the outside. For this reason, your cat will be carefully assessed at the Feline Centre before proceeding directly with treatment. It is important we identify any other problems that could change your or our minds as to whether your cat is suitable to receive radioactive iodine.

Once a cat receives radioactive iodine, he/she will be hospitalised in a special radioactive iodine ward separated from the main hospital building. If problems arise, they cannot be brought back into the main hospital due to radiation health and safety rules and we can only perform nursing treatment.

Based on our thirty years of experience, we consider this ‘geriatric MOT’ essential to minimise the risks as much as possible and only treat cats that are suitable.

It is a comprehensive assessment of your cat’s health. We will perform blood tests (unless these have been sent to an external laboratory within the last few weeks by your own vet), analyse urine and measure your cat’s blood pressure. Cardiac assessment by our cardiology specialists is an important part of our assessment to detect heart disease that may develop secondary to hyperthyroidism. We then perform imaging of the chest and abdomen under sedation to detect any concurrent diseases or complications arising from hyperthyroidism. Ideally, we try to assess your cat’s kidney function whilst the hyperthyroidism is well stabilised. Approximately 30-50% of hyperthyroid cats also have concurrent kidney disease, which may be hidden when the hyperthyroidism is uncontrolled. It is not unusual to unmask kidney disease by treating the hyperthyroidism but fortunately in most cases this is mild kidney disease that can be treated medically. However, it is important to treat the hyperthyroidism appropriately in this situation, to avoid further damage to the kidney.

We usually perform the radioiodine assessment 4-6 weeks in advance of a provisional treatment date. The situation can change quite quickly in older cats but this allows time for certain problems to be addressed, if needed, prior to treatment (eg. treatment of hypertension).

We prefer to do the assessment at the Feline Centre to be able to work with your cat, see how he/she settles in our hospital and develop an understanding of their personality. This allows us to collate all the results together and help guide you through the radioactive (RAI) treatment journey. We commonly find hyperthyroidism is not the only issue and performing a holistic assessment means we can prepare your cat for the best possible outcome with treatment.

The radioactive iodine must be ordered for a named patient and it is delivered two weeks later with optimal levels of radioactivity. It cannot be stored for any length of time, partly due to health and safety reasons, but it also undergoes radioactive decay and becomes less and less effective with time. This means we order RAI in advance, once we know a patient has been assessed and is suitable for treatment.

Sadly, this makes it almost impossible to combine an assessment and treatment visit as it would involve a very prolonged stay in the hospital. The purpose of our remote teleconsultation is to ensure we plan each visit carefully and ensure they are efficient and we address any concerns you have beforehand.

Your cat will be admitted 24-72 hours in advance of the planned treatment date, to allow him or her time to settle in and recover from the car journey. This will partly depend on how quickly they adapted to the hospital routine during their assessment visit. We obtain a baseline thyroid hormone (T4) measurement and recheck blood pressure. All thyroid medication will usually have been stopped one week in advance, as we need to establish exactly how high your cat’s thyroid hormone level is without treatment. We will advise regarding discontinuation of dietary therapy and transdermal medication on an individual basis.

The radioactive iodine injection is usually given on a Thursday or Friday, under a light sedation. Following this your cat is hospitalised in a special radioiodine ward and looked after by our dedicated feline nurses. Visits from the clinician in charge of your cat’s care are made as required.

The radioiodine ward has cattery-style accommodation with large pens, containing shelves at different levels. We provide various beds (including a cat fort), a radio and cat nip toys. We can feed whatever diet your cat prefers or requires, if he/she is receiving a special prescription diet. Please let us know in advance if your cat is on a special diet and we can ensure this is available. We have a variety of beds and types of bedding but please let us know if your cat has any special requirements.

We have webcams that allow us to check your cat from within the main hospital (unfortunately this is not accessible externally).

Unfortunately, this is not possible due to health and safety regulations. We are very happy to provide updates on a regular basis, via email or text messages. Our feline nurses are also happy to take photos and share these with you.

In our experience it is extremely rare for cats not to settle in the radioiodine ward. Once the injection has been given, the stay is no different to a routine cattery stay. This period can be harder for owners waiting at home than the patients - they are typically purring, content and happy in the ward!

The hospitalisation period for cats treated with standard and intermediate doses is currently 10 days. We determine the dose required based on your cat’s thyroid hormone level, severity of disease and thyroid tumour size when they are re-admitted for treatment. It is longer for cats being treated for thyroid carcinoma (4-7 weeks in total)

For the standard and intermediate treatment, cats can leave our hospital after 10 days provided the following restrictions can be observed (exact timings will be discussed by your clinician):

• maintain your cat indoors for approximately 2 extra weeks

• avoid contact with young children or those pregnant and breast-feeding for approximately 2 extra weeks

• avoid prolonged periods of direct contact with your cat for approximately 2 extra weeks (e.g. keep out of occupied bedrooms at night)

• handle waste with rubber gloves and double bag any waste before disposing of in general rubbish for approximately 3 extra weeks.

Our restrictions are set by a Radiation Protection Adviser, according to interpretation of Environmental Agency and local radiation regulations. Unfortunately, there is no accepted UK consensus as to when it is safe for a cat to return home and different hospitals may use different doses of radioiodine for treatment.

A minimum of 80% of cats have a completely normal thyroid level (T4) after 2 weeks. In others the T4 continues to drop over several weeks but it can take up to 6 months.

Approximately 5% of cats with benign disease may need a second treatment of radioactive iodine. This is much more likely in cats with extremely high T4 levels and chronic hyperthyroidism, where there may be a much larger volume of thyroid tissue. Thyroid medication reduces the T4 level but does not stop the continued growth of thyroid tissue. This means cats that have been hyperthyroid for a long time will slowly develop larger and larger thyroid glands, which is why we advocate treating with radioiodine earlier in the disease.

A second dose may also be required in cats with thyroid carcinoma. These patients need to be treated with substantially higher doses (up to 10 x higher) than a typical hyperthyroid patient. This dose is only given after a biopsy confirms that the disease is due to a thyroid carcinoma. We may also consider a higher dose in a cat that has failed to respond to a standard dose.

It is important to recognise that if a younger cat is treated, then there is the possibility that a new tumour could develop some time later e.g. after 2-3years. This is because we use a dose of radioiodine that preserves the normal (non-tumour) thyroid tissue and only damages the diseased tissue. If we used a higher ‘ablative dose’ to destroy all of the thyroid tissue present, then all cats would need to receive thyroid supplementation afterwards (as they would be hypothyroid).

Not necessarily - we would usually wait and monitor the cat’s T4 for several weeks-months, as the full effect of radioiodine may be delayed. We may suggest that your cat goes back onto thyroid medication (if it can be tolerated) in the interim to prevent adverse effects of uncontrolled hyperthyroidism. If it appears that a second dose is required, we will perform this at a reduced cost for you.

Many of the cats will leave the hospital with a low T4 level but it is rare for permanent hypothyroidism (inadequate levels of thyroid hormone) to develop post radioactive iodine. Usually the T4 levels increase to normal again after a few weeks-months. We use the lowest effective dose of radioactive iodine to try to prevent this happening, particularly as we know that any concurrent kidney disease could be affected by a hypothyroid state. There are two situations where we will recommend thyroid hormone supplementation:

1. If your cat’s kidney blood tests have significantly deteriorated compared to pre-treatment

2. If a very low T4 persists after several months; we will discuss some extra tests with your local vet to confirm hypothyroidism and look for any consistent signs (eg. lethargy, weight gain, greasy coat).

Supplementation is easy and well tolerated - there are liquid and tablet forms available and these are not associated with side-effects like the anti-thyroid drugs. Obviously, our aim is to have a successful result without the requirement for your cat to need any medication in the longer term. However, hypothyroidism is an easier situation to manage than severe chronic hyperthyroidism and it is important to remember that your cat’s tumour has been effectively treated.

We usually recommend a check-up with your local vet 3, 6 and 12 months after radioiodine, to perform blood tests, urinalysis and measure blood pressure. The purpose of these visits is to detect unmasking of kidney disease and high blood pressure, which could be a consequence of fully controlling the thyroid hormone level. Thereafter, senior checks are recommended to monitor kidney function and for hypothyroidism (low thyroid levels).