Hyperthyroidism happens when the body produces too much thyroid hormone. It’s a common cause of illness in middle-aged to older cats. Around 1 in 10 cats of any breed or sex will develop hyperthyroidism. If the condition is effectively managed, they can go on to enjoy a good quality of life for years.

Hyperthyroidism happens when the thyroid glands become overactive. The most common cause is a harmless growth of one or both thyroid glands. Early warning signs include weight loss, despite eating well and changes in behaviour. There are multiple treatment options available. In over 95% of cases, hyperthyroidism can be treated medically or even cured. So the outlook is good for most treated cats.

What is hyperthyroidism in cats?

Hyperthyroidism means there’s too much of the thyroid hormone, thyroxine, in your cat’s system. Thyroxine controls metabolic rate or how quickly the body burns up energy.

Your cat has 2 small thyroid glands in their neck. Over 95% of hyperthyroidism cases result from a growth on a thyroid gland. This growth is usually benign, meaning it doesn’t spread to other parts of the body. But it produces extra thyroid hormone. And thyroid hormone has profound effects on your cat's metabolism. Think of metabolism as a car’s acceleration. Hyperthyroidism is like having the accelerator pedal pressed down and stuck to the floor! Everything ‘speeds up’ uncontrollably and the engine burns through much more fuel (food and oxygen). Eventually, various ‘engine components’ (organs) will become damaged and stop working. And the ‘engine’ starts to fail. Inside your cat’s body, this means:

• Increasing nutritional (food) requirements to fuel increased metabolism

• Increasing heart rate and thickening heart muscle

• Increasing blood pressure

• Damage to kidneys and intestine

Hyperthyroidism is a serious illness, but a range of options exist for management and treatment.

Around 1 in 20 cases of hyperthyroidism result from cancer of the thyroid gland. Outlook is more uncertain for this group, although treatment options are available.

What are the symptoms of hyperthyroidism?

Symptoms of hyperthyroidism relate to increased metabolism, including the increased demand for fuel and the damage to vital organs.

• Losing weight despite eating more

• Vomiting and diarrhoea

• Drinking more and peeing more

• Becoming hyperactive, restless and/or more vocal

• Developing a dull, staring coat

• Increasing heart rate

• Breathing difficulties

Are some cats more at risk of hyperthyroidism?

Cats over 10 years old are most likely to develop hyperthyroidism.

All breeds and sexes are equally at risk.

Are my family or other pets at risk of hyperthyroidism

Hyperthyroidism affects individual cats. Your family and other pets are not at risk of getting hyperthyroidism.

However, medicine that suppresses your cat's thyroid hormone levels can have the same effect on humans.

• Medicine for hyperthyroidism must be handled carefully, especially by women of childbearing age

• Wash your hands carefully after handling tablets

• Clean surfaces thoroughly after any contact

• Keep all medicines out of sight and reach of children and pets.

How do vets diagnose hyperthyroidism

Vets diagnose hyperthyroidism based on the following:

• The ‘history’: any changes you report, e.g. losing weight, hyperactivity, vomiting etc

• Physical examination: sometimes vets can feel an enlarged thyroid gland in your cat's neck. This is called a goitre.

• Blood tests: general check and thyroid hormone level

What is the treatment for hyperthyroidism?

Four treatment options are available:

• Anti-thyroid medicines

• Anti-thyroid diet

• Surgery

• Radioactive iodine injection

The choice for your cat depends on the severity of their symptoms and the practical considerations. Practicalities include:

Will your cat take medicine?

• Will your cat take tablets?

• Does your lifestyle permit giving your cat medicine regularly?

• Does your cat have other health conditions, likely to be complicated by more medicines, anaesthetics or hospital stays?

• Does your cat go outdoors or hunt and do you have more than one cat?

Where you live

• Are you close to a clinic that offers surgery or radioactive iodine treatment?

• If not, does your cat travel well?

• Do you have transport?

Cost of treatment

Costs will vary depending on the options, the area you live and where you buy medication or special diets.

Variable costs for complications include:

• Side effects like vomiting and diarrhoea

• Anaesthetic reactions or post-surgery complications, including wound infections, hypothyroidism, hypoparathyroidism (low blood calcium), repeat surgery for a second or subsequent growth

• Hospital stays for longer than quoted

How to look after a cat with hyperthyroidism at home

There are no home remedies for cats with hyperthyroidism. But home support is essential to ensure your cat gets the most benefit from their vet treatment.

• Give thyroid medicine as prescribed at the correct dose and time

• Make sure your cat takes their medication (no spitting it out behind the sofa!)

• Ask for help or alternatives if you are struggling to give tablets

• Check your cat’s weight regularly. Weight loss can be the first warning sign that treatment needs reviewed

• Take your cat for regular vet checks and keep on top of preventive care, like worming and vaccinations

• Unless your cat is on Y/D prescription food, feed a diet which provides lots of calories (energy), enough fat and high-quality protein such as chicken, rabbit or fish

Can hyperthyroidism be prevented?

Hyperthyroidism can’t be prevented. But it can be controlled for long periods. It can even be cured with radioactive Iodine treatment

It's not always easy to notice gradual changes in your cat's weight and body condition when you see them every day.

• Weigh your cat regularly as they get older; every 1-3 months

• Weight Loss is an excellent early warning for ill health in cats

• Seek vet advice promptly if you do notice changes in your cat’s eating or drinking habits or behaviour

When to worry about cats with hyperthyroidism

Severe or untreated hyperthyroidism leads to organ failure and death. Vital organs such as the liver and kidneys are susceptible. But high blood pressure can also damage the eyes and brain.

Warning signs of worsening thyroid illness include:

• Persistent vomiting or diarrhoea

• Weakness and collapse

• Breathing difficulties- fast shallow breathing, open-mouthed breathing

• Lethargy (tiredness)

• Going suddenly blind

• Confusion, pacing, head-pressing

• Fits or seizures

Call your nearest vet if your cat is having seizures or suddenly seems blind

Thyroid treatment and kidney disease

Untreated hyperthyroidism will cause kidney damage.

But older cats are also at higher risk of kidney disease for other reasons. And symptoms of kidney disease may only start to show up after your cat starts antithyroid treatment. This happens because of the following:

• Hyperthyroidism causes increased blood pressure

• Higher blood pressure in untreated cats pushes blood into the kidneys

• As blood pressure falls in treated cats, the kidneys have less effective blood flow.

• Kidney impairment which was compensated by better blood flow may rapidly worsen

• Worsening kidney function complicates longer-term recovery and may make some treatments unsuitable.

Assess quality of life

• Recognising symptoms of hyperthyroidism

• Choosing and keeping to the right diet for your cat

• Giving medication

• Choosing the treatment option best for you and your cat