My cat has been diagnosed with hyperthyroidism, what is it?

Hyperthyroidism is the most common endocrine (hormonal) disease of cats. It is caused by an excessive amount of thyroid hormone in the circulation. The disease typically occurs in cats older than 8 years of age and can affect up to 10% of the feline population. In 97% of cases, the cause of hyperthyroidism is one or more benign (non-cancerous) nodules within the thyroid gland. The nodules produce thyroid hormones in excess irrespective of the body's needs, causing a range of clinical signs over time. Only in 3% of cases the cause is a malignant tumour of the gland called carcinoma.

What are the clinical signs of hyperthyroidism in cats?

Clinical signs in hyperthyroid cats will vary with the severity of the disease (i.e. the amount of thyroid hormones produced) and the time that has lapsed between developing the disease and the diagnosis (i.e. the longer the time to diagnosis the more noticeable the clinical signs). The typical clinical signs seen in hyperthyroid cats are progressive weight loss in the face of an increased appetite. Other common clinical signs include excessive drinking and urination, behavioural changes such as restlessness, nervousness or hyperexcitability, vomiting, diarrhoea or increased volume of faeces. Other clinical signs include fast breathing, open mouth breathing, coat changes such as hair loss, dull coat and unkempt coat, and nail overgrowth. Rarely hyperthyroidism will manifest with different signs such as tiredness and poor appetite.

How is the disease diagnosed?

Fortunately, diagnosing hyperthyroidism in cats is now relatively simple due to the existence of a reliable screening blood test. This test measures the circulating blood levels of total thyroxine (also called total T4), which is increased in 90% of hyperthyroid cats. In the remaining 10% of cases the total T4 levels are high-normal and other tests can be used to confirm the disease if the suspicion is high. It is recommended that all cats older than 8 years of age have total T4 measured at least once a year with or without clinical signs (as often early in the disease clinical signs are mild and can be missed).

Can my vet tell if my cat is hyperthyroid?

In addition to the typical clinical signs, most hyperthyroid cats will have an enlarged thyroid gland (called goitre), which can be appreciated on examination of the neck region performed by qualified veterinary surgeons. Other physical examination findings seen in hyperthyroid cats include poor body condition, fast heart rate and fast breathing rate. Some hyperthyroid cats will be hard to restrain on the examination table and can become unusually aggressive. These signs are due to the effect of excessive thyroid hormones on the brain.

Is it true that all hyperthyroid cats should have other blood tests, urine tests and diagnostic imaging performed at the time of diagnosis? If so, why?

Yes it is very true. It is recommended that all cats diagnosed with hyperthyroidism are screened (staged) for the presence of complications related to the disease such as secondary heart disease or failure, or urinary infection, or the presence of concurrent diseases typical of geriatric cats such as chronic kidney disease/failure or high blood pressure. Therefore all cats should have complete bloodwork, urine tests and blood pressure measurement. In some cases, a heart scan performed by a specialist cardiologist might be required. Diagnosing such complications and concurrent diseases is very important as it allows these diseases to be treated and this further testing also helps to determine what the best treatment would be for the hyperthyroidism. In fact, treatment options are different and some will be more suitable than others according to the results of this staging process.

Can my local vet do the staging?

Yes, it is possible for the staging to be done at your vets. However, it is advisable that your vet contacts a specialist in veterinary internal medicine to discuss the best treatment and monitoring options for your pet. In some cases, a heart scan is required to rule out the presence of heart failure therefore referral to a specialist veterinary cardiologist and medicine clinician might be recommended.

What are the available treatment options for hyperthyroidism in cats?

There are four treatment options for hyperthyroid cats. These are divided into definitive and non-definitive options according to whether they aim to cure (definitive) or simply manage the disease (non-definitive). Definitive options include radioactive iodine therapy (the gold standard treatment option) and surgery to remove one or both thyroid glands (unilateral or bilateral thyroidectomy). Non-definitive treatment options include anti-thyroid drugs such as methimazole and carbimazole, which block the production of thyroid hormones (but do not affect the overacting nodules), and a prescription diet with limited iodine content (iodine is required to produce thyroid hormones therefore by exclusively feeding a diet with very low iodine content the thyroid nodules cannot produce the hormones).

Please note that it is strongly recommended that all cats about to undergo definitive treatment with radioactive iodine or surgery receive a trial treatment period of at least 1-2 months with anti-thyroid drugs or, if not tolerated, with an iodine-restricted diet. This will allow the detection of severe chronic kidney disease in some cats, which may represent a contra-indication to definitive non-reversible treatment options.

What is the best treatment option for hyperthyroidism in cats?

All treatment options have advantages and disadvantages (see Table 1 below). In addition, it should be said that not all hyperthyroid cats are the same and, based on the stage of their disease, or the presence of complications or other concurrent debilitating diseases, one treatment modality might be better suited than others. However, for the majority of cases radioactive iodine therapy is considered the gold standard treatment of hyperthyroidism in cats, exactly like it is in people.

Furthermore, as previously mentioned, all cats diagnosed with hyperthyroidism should undergo a treatment trial of 1-2 months with anti-thyroid drugs or iodine restricted diets. This will allow the detection of severe kidney disease in some cats in which case radioactive iodine or surgical options may not be indicated. It is estimated that nearly 50% of hyperthyroid cats have kidney disease, which is masked by the effects of the thyroid hormones on the kidney function and therefore becomes apparent only after treating the hyperthyroidism.


Treatment option Advantages Disadvantages
Radioactive iodine (gold standard)
  1. Cure rate of 95%
  2. Single treatment in most cases
  3. No anaesthetic or surgical risks
  4. Rapid effect
  5. Definitive treatment
  1. Requires authorised facilities
  2. Long hospitalisation required to reduce radiation exposure to humans
  3. Can cause iatrogenic hypothyroidism
  4. Re-treatment possible in 5%
  5. Irreversible effects on GFR – not recommended in cats with significant azotaemia
  1. Usually corrects hyperthyroidism
  2. Relatively inexpensive
  3. Minimally invasive
  4. Rapid effect
  5. Definitive treatment
  1. Anaesthesia risks in elderly, fragile cats
  2. Risks of iatrogenic hypoparathyroidism
  3. Iatrogenic hypothyroidism with bilateral thyroidectomy
  4. Recurrence likely when unilateral surgery (bilateral disease in 70% of cases)
  5. Irreversible effects on GFR – not recommended in cats with significant azotaemia
Anti-thyroid drugs
  1. Usually corrects hyperthyroidism
  2. Inexpensive
  3. Available in tablets, liquid or topical gel
  4. No anaesthesia or surgery risks
  5. No hospitalisation required
  6. Effects on GFR reversible
  1. Side effects of drugs:
    1. Anorexia
    2. Vomiting
    3. Skin lesions
    4. Bone marrow suppression
    5. Live toxicity
  2. Daily or twice daily ongoing
  3. Ongoing costs of therapy and monitoring > surgery or RAI
  4. Iatrogenic hypothyroidism possible (monitoring required)
  5. Does not cure disease (not definitive treatment)
Iodine restricted diets
  1. Usually corrects hyperthyroidism
  2. Inexpensive
  3. No anaesthesia or surgery risks
  4. No hospitalisation required
  5. Effects on GFR reversible
  1. Rarely causes hypothyroidism –ongoing monitoring required
  2. Need to be fed exclusively and long-term
  3. Not ideal in multi-cat households
  4. Cats must be kept indoors to avoid outdoor sources of iodine
  5. Palatability issues
  6. Not ideal in cats with food responsive concurrent diseases
  7. Does not cure disease (not definitive treatment)
  8. Long-term consequences of iodine restriction not known


How does radioactive iodine therapy work?

A small amount of radioactive iodine (I131) is injected in the scruff of the neck just like a vaccination by a qualified veterinary surgeon. The overacting thyroid tissue takes up most of the I131 as it prepares to make thyroid hormones. Here the I131 emits beta particles (radiation), which will destroy the abnormal tissue while sparing all other organs. The radioactivity is very local and there is no hair loss or other side effects that are associated with radiotherapy for other forms of cancer.

Can my cat come home after the radioactive iodine injection?

No. After the treatment your pet will need to be housed in an isolation ward for health and safety reasons. This is because the amount of I131 that is not taken up by the thyroid is eliminated rapidly via urine and faeces. This means that urine and faeces of treated cats (as well as their thyroid) are radioactive and pose a small radiation exposure risk for staff members and owners.

During the isolation period, only minimal handling from qualified veterinary nurses is allowed twice a day (morning and evening). This time is sufficient for your pet to be fed and for their litter to be removed and their bed cleaned and to ensure that your pet is well and happy.

When can my cat come home?

You cat can come home as soon as the levels of radioactivity in urine and faeces have dropped to a safe level. This period is typically between 9-13 days after the injection. Will I be able to visit my cat during the isolation period? Unfortunately not. Visits are strictly prohibited due to radioactivity hazards. However, qualified veterinary nurses will regularly update you during your cat's stay.

What is the success rate of radioactive iodine therapy? Will my cat be cured?

Up to 95% of cats treated with radioactive iodine are cured of their hyperthyroidism. A small proportion of cats will need a second treatment six months later in order to be cured (this might be the case for cats with severe disease and very high levels of circulating thyroid hormones). An even smaller proportion of cats will fail treatment because they suffer from a malignant tumour of the thyroid gland called carcinoma, which requires much higher (ten times more than normal) doses of radioactive iodine. The latter cases are typically identified during the initial assessment or during treatment, as they require very high doses of medications to control the disease or are not controlled despite such high doses.

What will happen after discharge? Will I need to use any precautions and if so, how long for?

Your cat will be checked before being released home to ensure that it is safe for you to have him/her back home with you. However, some simple precautions will need to be taken for a further two weeks after discharge.

All owners of treated cats are obliged to follow such precautions and will be briefed about these at the time of discharge. They will be asked for signed proof of acknowledgment.

Once home your cat will need to be housed strictly indoors and to use a litter tray for approximately two weeks. During this time only limited contact between you and your cat will be allowed (i.e. 15 minutes on your lap once a day). Face-to-face contact should be avoided. Your cat should have no access to bedrooms or kitchen surfaces where food is prepared.

Children less than 12 years of age, and pregnant women (or those trying to conceive) should have no contact at all with your cat for this two week time period.

The litter containing urine and faeces should be handled with gloves and after placing into two plastic bags, should be placed into an outdoor waste container and stored for three weeks before being suitable for normal waste collection. After the two week period your cat will be free to go outdoors and be treated as normal.

How much does radioactive iodine treatment at Anderson Moores Veterinary Specialists cost?

The cost of the treatment is £1900 inclusive of VAT. This fixed fee includes a pre-assessment appointment (but excludes bloods, echo etc. if necessary at the pre-assessment).  Entry consultation and physical examination, blood pressure measurement, bloodwork including thyroid hormone levels, urine tests, the radioactive iodine and up to 13 days of hospitalisation post treatment. Please note that this price does not include any diagnostic imaging (e.g chest X-rays, heart or abdominal scans) which may be required in a small number of cases. The cost does not include any other treatments required during hospitalisation although this is not applicable to most cats. 

A deposit of £500 will be payable at the time of reserving your cats place for treatment. This is non-refundable although with enough notice the treatment date can be changed. The remainder balance will be payable at the time of admission to the hospital.

Will any monitoring be required after the radioactive treatment?

Yes, in order to ensure the success of the treatment and rule out the development of hypothyroidism (underacting thyroid) or the unmasking of pre-existing chronic kidney disease we recommend performing bloodwork including thyroid hormone levels, urine tests and blood pressure measurement at 30, 90 and 180 days after treatment. This can be performed at Anderson Moores Veterinary Specialists or at your local veterinary practice. Please note that the cost of these tests is not included in the cost of treatment set out above.

A very small proportion of cats will develop persistent hypothyroidism after radioactive iodine treatment and will need to receive thyroid hormone supplementation. The latter is easy to administer (in liquid and tablets form) and is safer in comparison to anti-thyroid drugs.



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