Feline Hyperthyroidism FAQ for owners

Frequently Asked Questions

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.

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.

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).

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.

Ideally, yes. 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 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. Also, further testing will help to determine what the best treatment would be for your pet. In fact, some treatment modalities will be more suitable than others according to the results of this staging process.

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.

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.

All treatment options have advantages and disadvantages (see table 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.

Treatment option
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. 9-14 days hospitalisation required to reduce radiation exposure to humans
  3. Rarely causes iatrogenic hypothyroidism (underactive thyroid gland)
  4. Re-treatment possible in 5%
  1. Usually corrects hyperthyroidism
  2. Rapid effect
  3. Definitive treatment
  1. Anaesthesia risks in elderly, fragile cats
  2. Some cases need two surgeries
    (disease is both sided in 70% of
  3. Hypothyroidism with bilateral thyroidectomy
  4. Risk of postoperative complications (low blood calcium, damage to the larynx and bleeding).
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
  1. Side effects of drugs:
    a. Loss of appetite
    b. Vomiting
    c. Skin lesions/sores
    d. Bone marrow disease
    e. Liver toxicity
  2. Daily or twice daily medication
  3. Ongoing costs of therapy and monitoring
  4. Hypothyroidism possible (monitoring required)
  5. Does not cure disease and therefore risk of development of thyroid cancer
Iodine restricted diets
  1. Usually corrects hyperthyroidism
  2. Inexpensive
  3. No anaesthesia or surgery risks
  4. No hospitalisation required
  1. Rarely causes hypothyroidism – ongoing monitoring required
  2. Need to be fed exclusively and long-term, with no access to any other food source
  3. Not ideal in multi-cat households
  4. Cats must be kept indoors
  5. Palatability issues
  6. Not ideal in cats with food responsive concurrent diseases
  7. Does not cure disease
  8. Long-term consequences of iodine restriction not known

A small amount of radioactive iodine (I131) is either injected in the scruff of the neck just like a vaccination by a qualified veterinary surgeon or an oral capsule is administered. If an injection is given, a small patch of fur will be clipped over the injection site. 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. This treatment can only be given at a very limited number of centres in the UK.

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 and veterinary surgeons is allowed twice a day (morning and evening). This time is sufficient for your pet to be fed, their litter to be changed and their bed cleaned, to ensure that they are well and happy.

Your cat can come home as soon as the levels of radioactivity in urine and faeces have dropped to a safe level. This is normally 8 days but very occasionally is 12 days after treatment. 

Unfortunately not. Visits are strictly prohibited due to health and safety reasons. However, qualified veterinary nurses will update you regularly during your cat’s stay.

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.

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. You can read this flyer to learn more about radioactive treatment.

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

There are two options for when to collect your cat after treatment with radioactive iodine:

1. Discharge on day 8 after treatment

In >95% of cases, levels of radiation are low enough just nine days after the treatment, for your cat to be safely allowed home. Radiation levels will be checked on day eight and our staff will call you to confirm the discharge appointment for the following day. Cats discharged on this earlier date will have slightly stricter precautions to follow at home and these will need to be followed for 17 days from the time of discharge. On the very rare occasion that radiation levels are still too high on day nine, your cat will need to stay hospitalised until the later discharge date.

The following precautions apply:

  • Cats are transported home in a suitable carrier, placed securely in the rear of a car.
  • Cats must not be taken on public transport.
  • Cats must be confined indoors with a litter tray available at all time.
  • Cats are not allowed in bedrooms or any areas where food is stored/prepared.
  • Cats should not be allowed contact with unworn clothing/bedding/towels etc.
  • Close contact with your cat (within 3 metres) must be restricted to a maximum of 15 minutes per day. Face to face contact must be avoided. Avoid stroking your cat on the head and do not allow your cat to lick your skin.
  • Wash your hands with soap and water after contact with your cat, litter tray and other potentially contaminated items (e.g. food bowls, other cats within the household).
  • Children under the age of 12 years, pregnant or breast-feeding women, or those trying to conceive should have no close-contact whatsoever with your cat, their litter tray or other potentially contaminated items.
  • Single-use, disposable gloves must be worn when emptying the litter tray.
  • Soiled litter should be double-bagged and sealed, before storing in a secure outside storage area (garage, shed) for three weeks after production, before normal disposal.
  • Your cat’s bedding should be washed no more regularly than once per week and should be washed at the end of the restricted period (17 days).
  1. Discharge on day 12 after treatment

When radiation levels are too high for your cat to be discharged at 9 days or when owners prefer their cat to remain hospitalised a little longer or feel more able to adhere to slightly more relaxed precautions at home, cats can be discharged 13 days following treatment. Precautions will need to be followed for 14 days from the time of discharge.

  • Cats are transported home in a suitable carrier, placed securely in the rear of a car.
  • Cats must be confined indoors with a litter tray available at all time.
  • Cats are not allowed in bedrooms or any areas where food is stored/prepared.
  • Close contact with your cat must be restricted to a maximum of 15 minutes per day at arm distance (for example the cat sitting on your lap). Face to face contact must be avoided.
  • Wash your hands with soap and water after contact with your cat or their litter tray.
  • Ensure that children under the age of 12 years remain at a safe distance from your cat and the litter tray.
  • Pregnant women or those trying to conceive should have no close contact with your cat or their litter tray.
  • Single-use, disposable gloves must be worn when emptying the litter tray.
  • Soiled litter should be double-bagged and sealed, before storing in a secure outside storage area (garage, shed) for three weeks after production, before normal disposal.

The cost for both periods of hospitalisation is the same. No extra fees are incurred for cats which stay with us for the full 13 days post treatment, whether this is due to owner’s preference, or due to radiation levels being too high for the cat to be discharged on the earlier date. After the applicable period of precautions, your cat will be free to go outdoors and be treated as normal.

Please note that your cat cannot be housed in a boarding cattery during the 14-17 day period of precautions

Yes, in order to ensure the success of the treatment and rule out the development of hypothyroidism (underactive 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. The 30-day assessment would ideally be done at Anderson Moores with all further follow up done at your local practice. Please note that the cost of these tests is not included in the cost of
treatment set out above. A 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 or tablets form) and is safer in comparison to anti-thyroid drugs. 

No, only a few veterinary centres in the UK have been authorised by the Environmental Agency to administer this treatment and to care for treated cats. Some of these centres are not managed by recognised Specialists in Internal Medicine. The Feline Hyperthyroid Clinic at Anderson Moores Veterinary Specialists is one of only eleven authorised centres in the UK and one of only five in the South of England. The clinic is run by a team of British, European and American Veterinary Specialists assisted by a team of highly qualified vets and nurses and a dedicated team of receptionists. 

While we welcome enquiries about radioactive iodine treatment from owners of hyperthyroid cats, your cat will need to be referred by your vet. They can contact the main hospital for details or forward a written request for referral to fhc@andersonmoores.com. Once our Internal Medicine Specialists have assessed the clinical history, our reception team will contact you to arrange an appointment and will be able to answer any further questions you may have. If our Specialists feel that radioactive iodine treatment might not be in the best interest of your cat they will contact your local vet to discuss the reason why in more detail.

Should you have any further questions regarding feline hyperthyroidism or radioactive iodine treatment, please do not hesitate to contact the Feline Hyperthyroid Clinic team at AMVS via email fhc@andersonmoores.com.