Hyperthyroidism and your cat

What is Hyperthyroidism?

Caused by enlarged thyroid glands which produce too much of the hormones T3 and T4

It is the most common hormone related condition of older cats. The majority are over the age of 10 at diagnosis.

More than 98% of enlarged glands are benign. If your cat does have a cancerous growth it may be harder to treat.

Symptoms you may see

· Ravenous appetite

· Weight loss

· Unkempt coat

· Drinking and urinating more

· Increased activity

Other symptoms your vet may see

· Increased heart rate

· Goitre - a lump in the neck

· Loss of muscle mass


To diagnose an overactive thyroid your vet will need to take a blood sample.

The blood sample will include a complete biochemistry which looks at the liver, kidneys and other parameters. We also look at a haematology sample to assess the red blood cell count and white blood cell count. An additional test to measure T4 is also needed to diagnose hyperthyroidism.

If left untreated

Without treatment the disease will progress and your cat will become increasingly thin.

Over time there are other effects on the body systems, the heart is affected and your cat may develop heart failure. There is often an increase in blood pressure which can damage the eyes, brain and kidneys.

Thankfully many of these changes can be reversed with treatment.


Medication - There is a variety of tablets, liquids and gels we can use to treat the symptoms. Treatment is lifelong and will require regular monitoring

Surgery - We can operate to remove the affected gland or glands. This can be curative but there is a possibility the condition may recur.

Radioactive Iodine - This is considered the gold standard treatment. Your cat is injected with a radioactive iodine which targets the overactive tissue and leaves the healthy tissue untouched. It is generally curative but as your cat will be radioactive they must be hospitalised in a specialist centre for up to 2 weeks. It can be very expensive.

Food - Without iodine the glands cannot produce hormone. If you use an iodine restricted diet then your cat can eat nothing else for the rest of its life. This includes anything they may hunt and eat.


Depending on which treatment path you chose there will be some level of ongoing monitoring. This will include multiple visits to the vets and repeated blood samples.

Treatment may uncover other conditions such as kidney disease which is another common condition in older cats.


It is possible you may be able to do something to help prevent your cat developing an overactive thyroid.

· Use pouches instead of cans of wet food

· Avoid soy, fish and giblet flavour foods

· Use ceramic or metal bowls

· Avoid cling film and plastic containers

· Use a natural, biodegradable cat litter

· Avoid fire retardants

This is not a guarantee that your cat will not develop an overactive thyroid but it may help and it won’t do any harm