Hyperthyroidism in cats: symptoms, diagnosis and treatment

25 October 2024 - 4 min read
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All mammals can suffer from thyroid issues. Typically, the two major issues they suffer from are hypothyroidism and hyperthyroidism. Hypo refers to the thyroid – a gland in a cat’s neck – producing too few thyroid hormones, while hyper refers to it producing too many thyroid hormones.

Hyperthyroidism is much more common in cats than hypothyroidism, and it can cause serious issues. Early treatment is essential, so you need to know the symptoms, risks, possible treatment plans and how it is diagnosed.

It can help lessen serious symptoms and help your cat get back to their best, healthy life. Find out all you need to know about hyperthyroidism in cats below.

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What’s hyperthyroidism in cats?

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Hyperthyroidism in cats is an endocrine disorder where the thyroid is overactive. This means it produces significantly more hormones than the body needs.

The thyroid is a gland in a cat’s neck (and ours too, funnily enough) that regulates essential processes in the body, like metabolism. Metabolism is the system that converts food into energy. The thyroid manages this with two hormones:

  • T3 - Triiodothyronine

  • T4 - Thyroxine

As you can imagine, when this gets out of balance, it can cause serious, widespread problems in the body.

In hyperthyroidism, the gland is overactive, meaning your cat is metabolising (burning) the energy in food too quickly.  This is more common in older cats and it is actually caused by a small benign tumour in the thyroid gland.

Symptoms of hyperthyroidism in cats

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Hyperthyroidism causes symptoms like:

  • Weight loss – the condition increases appetite, but no matter how much your cat eats, they’ll likely lose weight.

  • Increased thirst and urination – this is due to your cat’s metabolism working too quickly, impacting homeostasis.

  • Vomiting and diarrhoea – vomiting and diarrhoea can indicate a thyroid problem.

  • Restlessness – your cat may struggle to sleep and settle. They can seem hyperactive or more grumpy compared to normal.

  • Increased heart rate – you may notice your cat's heart rate when you hold them.

  • Greasy, poor-quality coat – your cat's coat will become greasy and feel different than normal.

  • Excessive vocalisation – cats aren't known to make their problems heard, but with hyperthyroidism, you can expect them to meow and make noise more often.

Untreated hyperthyroidism can lead to hidden problems such as an increase in blood pressure, as well as more visible ones like weight loss. This can affect your cat’s long-term prognosis, as well as their quality of life. You must take your cat to a vet if you suspect hyperthyroidism. 

Recognising symptoms early can lead to a timely diagnosis and treatment, which comprehensive cat insurance can cover.

Late-stage hyperthyroidism in cats

Late-stage hyperthyroidism is what happens when the condition doesn’t get treated. It’s where symptoms become more serious.

Late-stage hyperthyroidism complications can include:

  • Breathing problems

  • Collapse

  • Muscle weakness

  • Heart problems

  • High blood pressure

  • Kidney disease

  • Vision loss

Is hyperthyroidism in cats painful?

No, hyperthyroidism alone is not usually painful. But it does cause discomfort and stress, and it could also eventually be fatal if left untreated.

How is hyperthyroidism diagnosed?

Hyperthyroidism is diagnosed through a physical examination with your vet and a blood test to measure thyroid hormone levels.

This is usually run alongside a full blood panel that checks for other issues such as kidney and liver problems and anaemia.

Your vet may also suggest checking your cat’s blood pressure and a urine sample.

Hyperthyroidism in cats: treatment options

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Treatment depends on your cat’s lifestyle, temperament and any concurrent health issues.

There are four treatment options:

Anti-thyroid medication

Daily medication can be administered in the form of oral tablets or liquid. Your cat will require this medication for the rest of their life. This will need regular monitoring through blood tests to ensure that the dose is keeping your cat’s thyroid under control.

Diet changes

Iodine is needed to produce thyroid hormones, so putting them on a special iodine-restricted diet can control the problem. However, it only really works for indoor cats that live on their own, as eating even a small amount of normal food (including treats or prey from hunting) will undo the work of the diet. Your cat may require blood tests to monitor their response to the diet.

Radioactive iodine therapy

The administration of radioactive iodine to a cat can painlessly destroy the overactive tumour tissue within the thyroid gland. This is usually given at a specialist veterinary centre and your cat will need to be hospitalised in isolation for a short period after their treatment. The advantage of this type of treatment is that they are cured and will not require follow-up monitoring.

Surgical intervention (thyroidectomy)

Surgical removal of the abnormal half of the thyroid gland can be curative for some cats. This can cure the issue and means regular medication and monitoring is not required. But in some cases, the other side of the thyroid gland may become enlarged later and require further treatment.

What if my elderly cat is hyperthyroid?

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Elderly cats are much more likely to become hyperthyroid than younger cats. The treatment options listed previously will apply, and if you have good older cat insurance, you should get cover as long as the condition isn’t pre-existing.

If your cat is over 10, make sure you keep an eye on hyperthyroid symptoms. They’re more likely to get them at that age.

How to prevent hyperthyroidism in cats

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There’s currently no way to prevent hyperthyroidism and other thyroid problems. 

Early diagnosis is the best bet against hyperthyroidism in cats. The sooner treatment is started, the sooner your cat will start to feel better again.

Secondary issues such as blindness or kidney damage caused by elevated blood pressure will be less likely to occur. That’s why preventative pet care and regular check-ups are essential.

Understanding and managing thyroid issues in cats

So, hyperthyroidism is a complicated issue, but with regular vet visits, especially for cats over 10, and monitoring for common symptoms, you can stay on top of it.

As ever, you should stay as proactive with your cat’s health. We know they like to keep to themselves sometimes, but by monitoring for thyroid issues and other symptoms, you can get them the care they need quickly.

Cat insurance can help you stay proactive, with up to £15,000 vet fee cover and unlimited 24/7 vet calls with FirstVet.

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Dr. Rebecca MacMillan
BVetMed, BSAVA, PGCertSAM, MRCVS

Rebecca is a companion animal vet who has always had a passion for writing and client communication. Since her graduation from the Royal Veterinary college in 2009 she has gained a wealth of experience in first opinion small animal practice, in both clinical and managerial roles. She currently works in the South West and deals with a variety of routine and emergency appointments, but particularly enjoys medicine cases. Outside of work and writing, she enjoys spending time with her family, including her bouncy flat coated retriever George!