Hyperthyroidism in cats – signs and what to do

Updated 2026-09-30 · 2 sources

Based on: PDSA, International Cat Care (iCatCare)

Contact a vet today

Contact your vet today if you think your cat has this – it can become serious. It happens when one or both thyroid glands become enlarged and overactive, making too much thyroid hormone.

What to do now

What you can do now

Contact your vet today.
Note when it started, how often it happens and what else you see.
Take a photo or short video to show the vet.
Keep fresh water available.

Don't

Do not give any human medicine.
Do not wait if it gets worse.

What it is

Hyperthyroidism is a condition in which one or both thyroid glands become enlarged and overactive.^[1]^[2] It is serious and can be fatal if left untreated.^[1]

Signs

Weight loss
A good or increased appetite
Increased thirst
Increased activity, restlessness, or irritability
An increased heart rate
A poor, dry, or untidy coat
Drinking and peeing more
Sickness and diarrhoea
Low energy
One or two small lumps in the neck under the skin

Causes and risk factors

Enlargement of one or both thyroid glands

How a vet finds out

Physical examination
Blood tests
Urine tests
X-rays
Ultrasound
Blood pressure measurement

How vets treat it

Surgery
Hospitalisation
Diet change
Radiotherapy
Medication

What the experts say

“If your vet recognises the symptoms of hyperthyroidism it’s likely they will take blood samples to confirm. If blood tests show high thyroid hormone levels, hyperthyroidism will be diagnosed. Your vet may also want to check your cat’s kidneys and blood sugar.”
— PDSA [1]

“There are four main options for treatment, each with advantages and disadvantages: Radioactive iodine (I-131) is a very safe and effective treatment for hyperthyroidism. It has the advantage of being curative in most cats (up to 95%) with no ongoing treatment.”
— International Cat Care (iCatCare) [2]

If your pet is struggling to breathe, has collapsed, is having a seizure or can't stand up: go to the nearest vet now.

Not sure how serious it is?

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Common questions

What signs can hyperthyroidism cause in cats?

Signs can include weight loss, a good or increased appetite, increased thirst, increased activity, restlessness or irritability, a poor coat, and an increased heart rate.^[2]

How will a vet find out if a cat has hyperthyroidism?

A vet may take blood samples, and high thyroid hormone levels can confirm hyperthyroidism; they may also check the cat’s kidneys and blood sugar.^[1]

How may a vet treat hyperthyroidism?

Treatment options include medication to slow the overactive thyroid glands and surgery to remove the glands; radioactive iodine is also a treatment option.^[2]

Sources

How this page was made: written with the help of AI from the sources below, then checked claim by claim against those sources. Urgency levels follow OkPaw’s fixed safety rules. It is guidance, not a diagnosis. How we write our pages