Hyperthyroidism in Cats: Signs, Diagnosis and Treatment Options

At a glance

What it is	An overactive thyroid gland producing too much thyroid hormone, which speeds up the body's metabolism
Who's at risk	Mainly middle-aged and older cats; it's rare in young cats
Main signs	Weight loss with a big appetite, restlessness, night-time yowling, drinking more, vomiting, an unkempt coat, a fast heart rate
Contagious?	No
Can it be prevented?	There's no known way to prevent it, but regular check-ups help catch it early

What it is

The thyroid is a small gland in the neck, made up of two lobes that sit either side of the windpipe. It produces thyroid hormone, which acts a little like the body's accelerator pedal. It helps control how quickly the body burns energy, as well as heart rate, body temperature and many other processes.

In hyperthyroidism, the thyroid becomes overactive and produces too much hormone. In most cats this is caused by a benign (non-cancerous) enlargement of the gland. Very rarely, a cancerous growth is involved. Exactly why the gland changes isn't fully understood, and researchers continue to study possible factors.

With too much thyroid hormone, the body runs in overdrive. Cats burn through energy so fast that they lose weight even while eating more than usual, and the heart works harder than it should.

Signs to watch for

The signs usually develop gradually, and some can be easy to put down to age or personality. Common ones include:

• Weight loss despite a big appetite. This is the classic sign. Your cat may seem constantly hungry, beg for food or steal it, yet still get thinner.
• Restlessness or hyperactivity. Some older cats suddenly seem more energetic, unsettled or irritable.
• Vocalising, especially at night. Loud yowling or crying in the evening or early morning is common.
• Drinking and peeing more.
• Vomiting or diarrhoea. Eating quickly and in large amounts can contribute, and larger or looser stools are sometimes noticed.
• An unkempt or greasy coat. Some cats over-groom, while others stop grooming properly.
• A fast heart rate. You may notice your cat's heart pounding when you hold them, or breathing faster than usual.

A smaller number of cats show the opposite picture, becoming quiet, weak and off their food. Because weight loss and increased thirst also occur with other conditions, such as kidney disease and diabetes, it's important not to assume the cause. Our article on why your cat might be losing weight explains more, and the symptom checker can help you note what you're seeing.

When to see a vet

Book an appointment if your older cat is losing weight, seems hungrier than usual, is drinking more, is vomiting more often or has become restless or noisy at night. These signs aren't usually an emergency on their own, but they're worth checking soon.

Contact a vet urgently if your cat:

• is breathing rapidly, with an open mouth, or seems to struggle to breathe
• collapses or seems very weak
• suddenly can't use their back legs or appears to be in pain
• stops eating completely
• has repeated vomiting or diarrhoea and seems unwell

How vets diagnose it

Your vet will ask about your cat's signs and examine them. In many hyperthyroid cats, the vet can feel an enlarged thyroid gland in the neck. They'll also check heart rate, listen for heart murmurs or irregular rhythms, and weigh your cat.

The main test is a blood test to measure thyroid hormone levels. In most cats with hyperthyroidism, these are clearly raised. Sometimes the results are borderline, especially early on or if another illness is present, and your vet may suggest repeating the test or doing a more specialised one.

Your vet may also recommend:

• General blood and urine tests, to check other organs, particularly the kidneys.
• Blood pressure measurement, as high blood pressure can accompany hyperthyroidism.
• Heart checks, such as an ultrasound scan, if they hear a murmur or other changes.

Checking the kidneys is especially important. An overactive thyroid increases blood flow to the kidneys, which can make kidney tests look better than they really are. When the thyroid is brought under control, underlying kidney disease can sometimes become more apparent. Your vet will keep this in mind when planning treatment.

Treatment and management

The good news is that hyperthyroidism is usually very treatable. There are several approaches, and your vet will talk you through which might suit your cat, taking into account their age, other health conditions, temperament and your circumstances. In general terms, the options are:

• Daily medication. Tablets or other forms of medicine can reduce the amount of hormone the thyroid produces. This controls the condition rather than curing it, so treatment usually continues for life, with regular blood tests to check the balance is right.
• A special prescription diet. A diet that is very low in iodine, which the thyroid needs to make hormone, can help control levels in some cats. For it to work, the cat usually needs to eat only that food, which can be tricky in multi-cat or outdoor households.
• Radioactive iodine treatment. This is given at specialist centres and destroys the overactive thyroid tissue while generally sparing healthy tissue. It's often considered a cure for many cats, though it involves a hospital stay while the radioactivity falls to a safe level.
• Surgery. Removing the affected part of the gland can also be curative. As with any operation, it involves an anaesthetic, and your vet will consider your cat's overall health first.

Some cats start on medication first, which lets the vet see how the kidneys respond before a permanent treatment is considered. Please never adjust or stop a treatment without speaking to your vet.

Why treatment matters

Left untreated, hyperthyroidism tends to get worse over time. The constant strain can lead to heart disease, high blood pressure (which may damage the eyes and other organs) and further weight loss and muscle wasting. It can also affect the kidneys. Treating the condition can greatly improve how a cat feels.

Prevention

There's currently no known way to prevent hyperthyroidism. The most useful thing you can do is help your vet catch it early:

• Take your older cat for regular health checks, which may include blood tests.
• Weigh your cat regularly at home and note any changes.
• Mention new behaviours, such as night-time yowling or a sudden increase in appetite, even if they seem minor.

Living with it

Most cats with hyperthyroidism respond well once treatment begins, and many go back to being their usual selves. A few practical tips:

• Keep up with rechecks. Blood tests let your vet fine-tune treatment and keep an eye on kidney health. They're especially important in the first months after starting treatment.
• Track weight and appetite. Monthly weigh-ins and a simple diary help you and your vet see how things are going.
• Watch for changes. Tell your vet if your cat goes off their food, starts vomiting, becomes lethargic or develops itchy skin, as these may relate to treatment.
• Support general senior care. Our guide to caring for a senior cat covers comfort, diet and enrichment for older cats.

With good care and regular contact with your vet, many hyperthyroid cats enjoy a comfortable and happy life.