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Radioactive iodine treatment for hyperthyroid cats | Cat Specialist Services

Hyperthyroidism Explained / FAQ

Cat Specialist Hyperthyroidism
Cat Specialist Hyperthyroidism
Hyperthyroidism Images
Hyperthyroidism is caused by an overactive thyroid gland and is a common problem in cats over 10 years of age. It is the most common hormonal (endocrine) disease in cats and an important cause of illness in senior and geriatric cats around the world. More than 95% of cats are over 9 years of age.

Hyperthyroidism causes serious health consequences such as heart disease and weight loss. Common signs are increased hunger, weight loss, vomiting and behaviour changes such as crying and agitation.

Hyperthyroidism is a life-threatening condition, however it is treatable and curable in many
cases.

The gold standard treatment for cats (and humans) with hyperthyroidism is radioactive iodine treatment. This is the administration of a single oral capsule of radioactive iodine. This treatment has few, if any side effects and thyroid hormone levels are normal in 80% of cats by 4 weeks and 95% of cats by 6 months.

Below are some frequently asked questions about hyperthyroidism.
Q: What is the thyroid gland?
All mammals have two thyroid glands in the neck, one on either side of the wind pipe (trachea). In cats with hyperthyroidism, the gland/s enlarge and can be felt in the neck. A normal thyroid gland is about 4-8 mm wide and 10-20 mm long.
Q: What does the thyroid gland do?
Thyroid glands make thyroid hormones (e.g. T 4, T 3 ) which control metabolism i.e. how your body transforms the food you eat into energy.

Thyroid hormones affect every cell and organ in the cat’s body. They regulate how the body uses calories (the metabolic rate). In hyperthyroid cats, the metabolic rate is very fast causing weight loss despite cats eating a lot of food.

Thyroid hormones also speed up or slow down the heart rate, raise or lower body temperature and increase the speed food moves through the digestive tract.
Q: How many cats are affected with hyperthyroidism?
Hyperthyroidism was only reported about 40 years ago but the diagnosis of hyperthyroidism in cats has dramatically increased over the years. It is now the most common hormonal disorder in cats around the world.

Hyperthyroidism is more common in cats compared with other domestic animals. Although there has been much investigation it remains unclear what the underlying cause is.
Q: What causes cats to get hyperthyroidism?
We don’t know why cats develop hyperthyroidism more commonly than other species. Iodine deficiency or fluctuating dietary iodine levels or long term exposure to thyroid disrupting compounds (e.g. BPA, PBDEs) in the diet or environment all likely play a role. Cats maybe more susceptible to these agents as their exposure is increased through grooming and ingest.


Figure: Cats may ingest BPAs whilst grooming themselves.
Q: What happens in the thyroid gland of a cat with hyperthyroidism?

Hyperthyroid cats have abnormal thyroid glands with single or multiple nodules (hyperplastic and adenomatous nodules) of abnormal thyroid tissue. Over time, normal thyroid tissue changes to thyroid hyperplasia and then a thyroid adenoma. A thyroid adenoma is a type of benign cancer. Benign means it doesn’t spread elsewhere in the body.

Over time, the abnormal thyroid tissue continues to grow and can transform into a more malignant type of cancer (thyroid carcinoma). This is more common in cats treated for long periods with medical therapy such as methimazole.

A cat with large Thyroid mass

 

Q: What are the signs of hyperthyroidism?

Common signs are weight loss despite cat’s having an increased appetite and eating more. When diagnosed early, weight loss can be mild but becomes severe with time.  Hyperthyroidism is a common cause of weight loss in older cats Vomiting, hyperactivity, increased thirst,increased urination, diarrhoea and increased stool size may also occur.

Cats can show less interest in grooming and their nails can become very long and brittle.

Cat can loose weight grow claws

Q: What about high blood pressure (hypertension) in hyperthyroid cats?
High blood pressure (hypertension) may also be associated with hyperthyroidism. Approximately 10-25% of cats with untreated hyperthyroidism have high blood pressure although some studies have suggested this number is higher.

High blood pressure in cats has significant health impacts as it causes damage to other organs in the body such as the eyes, heart, brain and kidneys. Changes to the eyes (bleeding in the eye, detachment of the retina, degeneration of the retina) can result in vision impairment and blindness. Assessment of blood pressure is recommended in any senior cat, particularly those diagnosed with kidney disease or hyperthyroidism.


High blood pressure can result in changes to the eyes and blindness.
See here for more information on hypertension in cats.
Q: What sort of heart disease occurs with hyperthyroidism?
Thyroid hormones directly affect the heart causing thickening (hypertrophic
cardiomyopathy) of the heart muscle.

The heart beats much faster and a heart murmur or irregular heart rhythm occur in
approximately 20-30% of cats. With early diagnosis and treatment, these effects are
minimised.

Severe heart disease is associated with chronic, untreated hyperthyroidism or cats may
have underlying heart disease as well as being hyperthyroid.

If heart failure does occur in cats, signs include rapid, shallow or open mouth breathing,
inappetance and hind leg paralysis due to the formation of blood clots.
Q: How is hyperthyroidism diagnosed?
Hyperthyroidism is diagnosed by measurement of blood thyroid hormone levels. Measurement of Total T4 concentration is the most common test. An elevated level in combination with appropriate clinic signs is consistent with a diagnosis of hyperthyroidism.

Sometimes cats may have symptoms of hyperthyroidism, however, the thyroid blood test is within the normal range. Additional testing using other methods or repeating a Total T4 at a later stage maybe required to support the diagnosis.
Q: How do I know if my cat has heart disease associated with their hyperthyroidism?
The best way to assess a cat’s heart is by performing a heart ultrasound (echocardiogram). This provides comprehensive detail about the heart’s structure and function. Sometimes an ECG (electrocardiogram –assessing the electrical activity of the heart) and chest radiographs (x-rays) are required.

At CSS we are very fortunate to have on site daily access to the largest and most experienced team of veterinary cardiology specialists to ensure the highest level of care for your cat.
Q: How do I know if my cat has kidney disease when they are hyperthyroid?
Cats with hyperthyroidism are older cats who are also at risk of developing chronic kidney disease. It can be difficult to identify kidney disease in hyperthyroid cats as the metabolic effects of hyperthyroidism make kidney function appear better than it actually is.

Kidney function is assessed using blood tests (e.g. urea, creatinine, SDMA) and urine testing (urine concentrating ability). Hyperthyroidism affects all of these tests results.

The kidney function of a hyperthyroid cat can only be reliably assessed when a cat has had completely normal thyroid function for at least 3 months.
Q: What are the treatments for hyperthyroidism?
Hyperthyroidism is a progressive disease that is life-threatening if not treated.
There are four methods of treatment:
(i) Medical management: e.g. methimazole, carbimazole tablets or transdermal
ointment
(ii) Radioactive iodine treatment
(iii) Dietary management e.g. low iodine diet (Hills y/d)
(iv) Surgical removal of the thyroid gland (thyroidectomy)

Only radioactive iodine treatment and surgical removal of the thyroid gland can cure a cat of
their hyperthyroidism.
Q: What is the best treatment for my cat’s hyperthyroidism?
All treatments have advantages and disadvantages. The initial treatment recommended by your vet is based on a combination treatment availability, cost, how easy a cat to medicate and underlying diseases (e.g. chronic kidney disease) that the cat may have. It is important to have a detailed discussion with your vet about which is the best treatment option for
your cat.
Q: How does medical management of hyperthyroidism work?
Anti-thyroidal drugs interfere with thyroid hormone production. These drugs do not address the underlying cause of the hyperthyroidism e.g. the abnormal thyroid tissue.

Anti-thyroidal drugs are only effective when given continuously and are required for life. If medication is stopped, the cat will become hyperthyroid again very quickly.

The abnormal thyroid continues to enlarge, producing more hormone and so more medication is needed to keep the thyroid hormone level within normal range. This requires ongoing monitoring to make sure dosing is sufficient.

With medical management, over time there is a higher risk of transition of a thyroid adenoma to a thyroid carcinoma, a malignant form of thyroid disease that can be more resistant to treatment.

A gradual increase in thyroid tumour size is expected in cats receiving anti-thyroid drugs.


This image shows a cat with a large thyroid mass (blue circle) which is most
likely a thyroid carcinoma.
Q: What are the side effects associated with medical management?

Possible side effects from anti-thyroidal medication include gastrointestinal signs e.g. vomiting or diarrhoea, occuring in approximately 25% of cats. This may be less with transdermal ointment. These side effects are generally mild and resolve without needing to stop treatment.

Sometimes cats can develop an itchy face or crusty skin.

More severe, but rare side effects include liver disease and damage to the bone marrow resulting in low white cell counts (neutropenia, agranulocytosis) or platelet disorders (thrombocytopenia). Luckily these side effects are very rare. Blood tests are typically used to identify these changes and anti-thyroid drugs should be stopped if they occur.
 


Although rare anti-thyroidal drugs can cause adverse bone marrow effects causing anaemia. Note this cat has pale gums from anaemia.

If a cat has reacted to one drug (e.g. methimazole) they will likely react to the other drug (e.g. carbimazole) and other treatments should be considered.

Q: How successful is medical management to treat hyperthyroidism in cats?
Control of hyperthyroidism using medical management is possible for most cats with significant reduction in thyroid hormone within 4-6 weeks. The best outcome with medical management requires regular monitoring of thyroid hormone levels and dosage adjustments of medications for life.

With long periods of medical management (e.g. greater than 2-3 years) there is a higher risk of progression of a thyroid adenoma to a thyroid carcinoma – a more malignant cancer.
Q: What about surgical removal of the thyroid glands?
Surgical removal of one or both thyroid glands is possible and may be a permanent cure. Unfortunately it requires general anaesthesia, which can be risky in older patients with significant effects from their hyperthyroid state (e.g. heart disease) or with other diseases.

Some cats can have thyroid tissue in other places – not just the thyroid gland (e.g. the base of the tongue or in their chest) and this tissue cannot be surgical removed.

Surgery has a role to play in cats that cannot receive radioactive iodine but also cannot tolerate or receive medical management. Ideally one thyroid gland is removed at a time (I.e. two separate surgeries) to reduce the risk of complications. A recent study identified quite a high recurrence rate of hyperthyroidism (22%) in cats despite removing both thyroid glands (Covey et al JVIM 2018).
Q: What is radioactive iodine treatment and how does it work?
A single capsule of radioactive iodine (I 131 ) is given to your cat, who then absorbs it. The radioactive iodine travels to the thyroid gland where it irradiates and destroys abnormal thyroid tissue. The iodine is trapped just in the thyroid gland that uses it to make thyroid hormone, so it is the only tissue affected.

Generally, it is the abnormal thyroid tissue that “absorbs” the most radioactive iodine leaving the “normal” thyroid tissue.
Q: Why is radioactive iodine considered the gold standard for treating hyperthyroidism?
Radioactive iodine (I 131 ) provides a simple, safe and effective treatment avoiding difficulties of life-long daily drugs and monitoring, drug side effects or restrictions associated with feeding an iodine-restricted diet. Risks associated with anaesthesia and surgery are also avoided.

Radioactive iodine can cure hyperthyroidism in the majority of cats. Thyroid hormone levels are within normal range in 80% of cats by 4 weeks and 95% of cats by 6 months.
Q: Why do cats have to kept isolated with radioactive iodine treatment?
Handling and administration of radioactive iodine to cats requires special licensing, hospitalisation equipment and strict compliance with state radiation laws (Queensland Health).

Cats receiving radioactive iodine continue to excrete radioactivity in their urine, stools and saliva for a period of time dependent on the dosage of iodine administered (typically 2-4 weeks).

Radioactivity is significantly reduced within 5-7 days and cats can leave the radioactive unit at that time, however they must remain indoors for a further 2 weeks.
Q: How safe is radioactive iodine treatment? Is there any risks to my cat or me?
Radioactive iodine treatment is extremely safe for cats and their owners.

Radioactive iodine treated cats excrete small amounts of radioactivity in their urine, stools and saliva for 2-4 weeks. In order to reduce human exposure treated cats must be kept indoors only for 2 weeks after they have been discharged from the iodine unit and contact with their saliva, stools, vomit or urine avoided.

If these instructions are followed the risk from radiation to humans is minimal.
Q: I have other cats at home. Is there any risk to them?
We would suggest keep your cats separate for the 2 weeks after your cat has returned home and avoid them sharing a litter box and coming into contact with urine or stools.
Q: Am I allowed to visit my cat while they are in the radioactive iodine unit?
Unfortunately not. Administration of radioactive iodine requires special licensing and hospitalisation equipment and strict compliance with state radiation laws (Queensland Health). Don’t worry though, we have taken every effort to make sure that your cat will be as comfortable as possible and have a quiet and relaxing week.
Q: Can my cat have a t-shirt or something that will remind them of home for when they are in the radioactive iodine unit?
Absolutely. Please note though, we cannot return any of these items as they must be disposed of post-radiation to comply with state radiation laws. We also ask that the items are only small such as a t-shirt or pillowcase.

If your cat has a little pillow case or t-shirt they like to sleep on they are welcome to bring this with them into the iodine suite.
Q: Can my cat have their own food while they are in the radioactive iodine unit?
Absolutely. You are welcome to provide your cat’s own food supply. We keep many brands of cat food in hospital, however it always helps to have your cats favourite.
Q: I’m worried my cat will get bored. What sort of things can my cat do while they are in the radioactive iodine unit?
Hyperthyroidism is a disease of older cats, who lets face it – spend a lot of time sleeping. However, we have ensured that each cat will have direct sunlight, many sleeping places, ramps, toys and a garden view. We also have “cat tv” that is programmed to come on intermittently throughout the day.


All suites have visibility of a cat TV which will show different programs at various times throughout the day to help provide enrichment
Q: What do I have to do with my cat before they receive radioactive iodine treatment?
Consultations for assessing cats are available on a Monday or Tuesday. If you are unable to bring your cat at that time please contact our reception.

Hyperthyroidism affects older cats who may have other diseases impacting their safety and suitability for receiving iodine treatment. For example cats with significant, pre-existing kidney disease may not be suitable candidates for iodine.

Cats are ideally screened for underlying diseases prior to entering the unit. This screening is basic blood work, chest xrays and ideally abdominal ultrasound +/- a heart scan. This screening can be performed by your vets and results forwarded to us at least 7 days prior to their appointment or CSS can perform all pre-screening prior to treatment (see below).

Once we know your cat is safe to receive radioactive iodine treatment this is administered on a Wednesday or Thursday.

Any anti-thyroidal medications (e.g. carbimazole, methimazole, Hills y/d) needs to stop 14
days before treatment.
Q: What happens on the day my cat receives radioactive iodine treatment?

Cats are fasted overnight and receive anti-nausea medication (maropitant) to help reduce the risk of them vomiting the radioactive iodine capsule. They have a physical examination and body weight assessment recorded and are admitted into the radioactive iodine unit.

They are given the radioactive iodine capsule orally and board in the radiation suites for 5-7 days.

We aim to give the safest, lowest and effective dosage to each cat.

Q: What care does my cat receive while they are in the radioactive iodine suite?
Cats are checked in the radioactive iodine suites by a dedicated, highly trained feline nurse, supervised by a specialist feline veterinarian. Patients receive daily fresh food, water and litter changes and are monitored to ensure their radioactivity levels are reducing to a suitable level for discharge.

Once ready for discharge, cats can then move into our medical boarding suites until a discharge time can be arranged.
Q: What do I need to do for my cat when they come home after they have received radioactive iodine treatment?

Radioactive iodine treated cats excrete small amounts of radioactivity in their urine, stools and saliva for 2-4 weeks. In order to reduce human exposure it is a requirement that treated cats are kept indoors only for 2 weeks after discharge and you avoid contact with their saliva, stools, vomit or urine.

During this period we recommend:
(a) Avoiding long periods (more than a few minutes) of direct contact with your cat. It is safe
to pick them up for short periods, but they should not sit on anyone’s lap for extended
periods or sleep next to you on the bed.
(b) Avoiding “face-to-face" contact or allowing the cat to lick or groom humans.
(c ) Wash your hands after patting your cat.
(d) Avoid contact with urine, stools or vomit. Litter trays should be kept in an unoccupied
area and lined with a waterproof litter tray liner. All litter tray waste should then be
disposed of in a rubbish bag. Wear gloves when changing the litter and when cleaning any
excrement outside the litter box (e.g. vomit).

If these instructions are followed the risk from radiation to humans will be minimal.

Q: I don’t think I can keep my cat indoors for the 2 weeks after radioactive iodine treatment. What can I do?

Your cat would be welcome to stay for the 2 week period in our medical boarding suites. These suites are designed to be welcoming and comfortable for cats with direct sunlight, multiple shelves for lounging. Cats also spend time in one of three playrooms – Starwars, Harry Potter or the Lord of the Rings playrooms.

The cats are cared for in the suites by highly trained staff and supervised by specialist feline veterinarians. The medical boarding suites are located directly opposite our offices so when staff have “spare time” we can go and play and groom the cats. Trust us. We love cats.

Q: Can I visit my cat once they are in the medical boarding suites?
Yes, absolutely.
Q: What monitoring is required after they receive their radioactive iodine treatment?
Follow up blood tests are recommended between 6 and 12 weeks after treatment to assess kidney function and thyroid hormones levels. Thyroid hormone levels are within normal range in 80% of cats by 4 weeks and 95% of cats by 6 months.

These tests can be performed at your local vets and forwarded to us and we will help with any follow up required.
Q: Are there any side effects with radioactive iodine treatment?
Side effects associated with radioactive iodine treatment are very uncommon.

Hypothyroidism (an inactive thyroid gland and deficiency in thyroid hormone) is possible and more likely with higher radioactive iodine doses.

It tends to occur weeks to months after iodine treatment and may require supplementation with thyroid hormone. Hypothyroidism if it occurs can worsen kidney function and may shorten survival time. Avoiding hypothyroidism is important and is less likely to occur with lower doses of radioactive iodine.

In some cats with underlying kidney disease, when their thyroid level normalises this can unmask their kidney disease.
Q: What happens if my cat develops kidney disease after their radioactive iodine treatment?
Hyperthyroidism masks underlying kidney disease. Approximately 15% of hyperthyroid cats have increases in kidney blood tests after treatment, however this is often not associated with a worse outcome.
Q: Do all cats respond to radioactive iodine treatment?
Approximately 5% of cats don’t respond completely to radioactive iodine and remain persistently hyperthyroid. This is more likely in cats with very large tumours. Repeat treatment with radioactive iodine can be considered if cats are persistently hyperthyroid 6 months after initial treatment and if their symptoms have not improved.
Q: Can my cat get hyperthyroidism again?
Relapse of hyperthyroidism is very uncommon but is reported in approximately 5 % of cats treated. If relapse does occur it is generally after 3 or more years.
Q: How do I book an appointment for my cat to receive radioactive iodine?
Please add your details to our waiting list for Radioactive Iodine treatment. Our reception team will contact you to organise an appointment.
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