Understanding Feline Hyperthyroidism

If your vet has mentioned hyperthyroidism in your cat, you’re not alone — it’s one of the most common conditions seen in older cats. The good news is that it’s usually very treatable, and many cats go on to live happy, comfortable lives once diagnosed and managed properly.

What is hyperthyroidism?

Hyperthyroidism happens when the thyroid glands (small glands in the neck) produce too much thyroid hormone. These hormones help control metabolism — essentially how fast the body runs.

When too much hormone is produced, your cat’s body goes into overdrive. Everything speeds up, which is why affected cats often seem restless, hungry, and thinner than usual.

Why does it happen?

In most cases, hyperthyroidism develops because part of the thyroid gland becomes enlarged and overactive as cats get older. This is usually benign (non-cancerous) and we do not yet fully understand why it happens.

It tends to occur in cats over about 10 years of age and is seen in both males and females.

Common signs you might notice at home

Has your cat started acting like a kitten again? Many owners feel their cat has suddenly “aged backwards” — becoming more active or demanding food constantly — but at the same time losing weight.

Typical signs include:

• Weight loss despite a good or increased appetite
• Eating more than usual
• Increased thirst and urination
• Vomiting or occasional diarrhoea
• Restlessness or increased vocalisation
• A scruffy or unkempt coat
• Faster heart rate

Some cats show the opposite and become quieter or less interested in food — this is less common but still possible.

How is it diagnosed?

Diagnosis is usually straightforward and involves:

• A physical examination (your vet may feel an enlarged thyroid gland in the neck, but this isn’t always present)
• A blood test, which measures thyroid hormone levels

Because many hyperthyroid cats are older, your vet will often check kidney function and blood pressure at the same time. This helps us plan the safest treatment and look for other age-related conditions.

Treatment options

Treatment falls into two main categories – cure – radio-iodine or surgery or management – long term medications or a food restricted diet.

Radioiodine therapy

A one-off treatment of radio-iodine that targets overactive thyroid tissue.

• Considered the gold standard
• Usually cures the disease and works first time in 95-99% of cases
• Avoids daily medication, and can work out cheaper in the long term
• Given as an injection (similar to a vaccine) or a capsule
• Cats can only be treated at a few veterinary practices in the UK, and they have to stay in the practice for a few days afterwards until they can safely be sent home

Surgery

Removal of the thyroid gland is possible but there are some associated risks, such as accidental removal of the glands which lie next to the thyroid gland.

Your vet will discuss which option suits your cat’s health, temperament, and lifestyle.

Special iodine free diet

A prescription diet low in iodine can help control the condition, but must be fed exclusively. This option is therefore not appropriate for outdoor cats.

Medication

Tablets or a topical gel can reduce thyroid hormone production.

• Requires daily administration of a tablet or a liquid to affected cats
• Good for long-term management, but does not cure the disease
• There can be side effects so requires frequent blood tests. The manufacturers recommend blood tests at weeks 3, 6, 10, 20, and every 3 months thereafter after starting medication
• Requires regular monitoring and blood tests to check thyroid levels are within the normal range

What about the kidneys?

One important thing owners should know is that hyperthyroidism can sometimes hide underlying kidney disease. When thyroid levels are brought back to normal, kidney problems may become more apparent.

This sounds worrying, but it’s very normal — and one reason careful monitoring is so important during treatment.

What is the outlook?

The prognosis for hyperthyroid cats is generally excellent with appropriate treatment. Many cats regain weight, settle behaviourally, and enjoy a very good quality of life for years afterwards.

Regular follow-up checks help ensure treatment stays balanced and your cat remains comfortable.

When should you speak to your vet?

If your older cat is:

• Losing weight
• Eating more than normal
• Drinking or urinating more
• Becoming unusually active or vocal

…it’s worth arranging a health check and blood test to include thyroid measurement with your normal vets. Early diagnosis makes management easier.