15 Symptoms for Hyperthyroidism in Cats: Early Warning Signs of an Overactive Thyroid

Hyperthyroidism is a condition where the thyroid gland becomes overactive. The thyroid produces T3 and T4 hormones, controlling metabolism and how quickly the body uses energy. The condition is the most common hormonal disorder in cats, typically caused by a benign enlargement of the thyroid gland.

It is usually cats over 10 years old that are affected. Symptoms are sometimes missed as they are mistaken for “normal ageing” and can be quite subtle at first. However, early diagnosis improves potential outcomes, while untreated cases can lead to heart disease and organ damage.

In this guide, we will go over the most common symptoms of hyperthyroidism in cats to help you spot the signs early and get your pet to a vet.

15 Symptoms for Hyperthyroidism in Cats

When it comes to hyperthyroidism in cats, not all animals will display the exact same symptoms. Many cats show around 3-5 of them at first, and they often develop gradually over months. This is why it’s important to know the full range of symptoms and spot them as early as possible.

With that in mind, here are the top 15 symptoms that could suggest your cat has an overactive thyroid.

#1. Unexplained Weight Loss (Despite Eating Well)

This is a critical early sign of hyperthyroidism and is often the first thing people notice. It happens because your cat’s metabolism goes into overdrive, meaning they are burning calories faster than they can consume them.

If you notice significant weight loss that happens very quickly, alarm bells should be ringing. As the cat is effectively in calorie deficit, this can lead to muscle wasting along the spine and hind legs over time. You might notice a prominent backbone and hips.

Even overweight cats can quickly become underweight. The body condition score will drop noticeably. If you can see your cat losing weight but eating normally, this is a sign to get it to the vet for a checkup.

#2. Increased Appetite (Polyphagia)

This one connects to the unexplained weight loss. If your cat seems constantly hungry, it could be a sign that it is burning calories too fast. Things you might notice here include:

• Begging for food

• Stealing food from your plate

• Waking you earlier than usual for feeding

In these circumstances, it is not unusual for a cat to start eating double its usual intake of food. It happens because the metabolism is overactive. In later stages of the condition, the appetite may decrease again, but damage could already be done to the cat’s body.

#3. Increased Thirst (Polydipsia)

You may notice that your cat is drinking considerably more water than usual from its water bowl. They may also keep looking for water elsewhere, such as from taps, baths or even the toilet. This is an important diagnostic clue in older cats, and it could indicate more than just hyperthyroidism.

This increased drinking is often linked with increased urination, as you might expect. It may also overlap with symptoms of kidney disease. Sudden increased thirst in cats is rarely a good sign and should warrant a trip to the vet.

#4. Increased Urination (Polyuria)

Have you noticed larger than usual urine clumps in the litter tray? If your cat is going to the toilet more frequently and doing larger wees, this should not be ignored. You may also notice they are having accidents outside the tray.

This symptom is often confused with a sign of diabetes or kidney disease, but hyperthyroidism is actually a common cause of it. If you notice that your cat is drinking more than usual and urinating more frequently, tell your vet about it.

Incidentally, if you use Litter Sense cat litter, your cat’s urine becomes an indicator of a problem as the litter changes colour when it comes into contact with different hormones or pH levels that shouldn’t be present in a cat’s urine.

#5. Hyperactivity and Restlessness

Cats over the age of 10 tend to be more lethargic and relaxed than younger cats. If you notice your cat suddenly seems to have unusually high energy, this could be a symptom of hyperthyroidism. You may notice things like:

• Pacing

• Difficulty sitting for long periods

• Increased night-time activity

In essence, your cat may appear “younger” temporarily. It may seem exciting at first, but it could be a sign that something is wrong. With hyperthyroidism, it happens because excess thyroid hormone is stimulating the nervous system.

#6. Increased Vocalisation

Has your cat been meowing more than usual recently? Have you noticed it yowling at night? This may be amusing or annoying, but it could also be a sign that something is wrong and you shouldn’t simply ignore it.

Persistent attention seeking is unusual behaviour in some cats, while for others it is quite normal. If it is a behavioural change for your pet, it could be a sign of cognitive decline. But it could also be a symptom of an overactive thyroid, linked to agitation and hypertension.

#7. Poor or Unkempt Coat Condition

Have you noticed that your cat’s fur is starting to look greasy or matted? Perhaps you observe a dull coat when you look at your cat, where it once looked vibrant and shiny. Reduced grooming can lead to this, and it is quite unusual for cats to cease grooming their fur.

Coat changes like this often accompany weight loss. If you notice patchy or thinning hair on your cat, it could be a sign of several things, one of which is an overactive thyroid. Book an appointment with your vet and get it checked out.

#8. Vomiting

This is a symptom that could be a sign of many problems in your cat. With hyperthyroidism, it is often linked with an increase in appetite. This combination of symptoms may be caused by metabolic imbalance and GI hypermobility.

Often, vomiting is not a cause for concern in cats. However, if it is occasional to frequent, happening several times per week, you should pay attention. Report the symptom to your vet and they will probably recommend that you make an appointment.

#9. Diarrhoea or Loose Stools

When a cat has hyperthyroidism, it may suffer from increased gut motility. This can lead to issues with stools whereby they come out softer than normal. You may also notice that your cat is defecating more frequently than usual.

In some cases, the diarrhoea may alternate with vomiting. These are symptoms that could suggest a number of possible conditions, so we recommend you get your cat checked out.

#10. Rapid Heart Rate (Tachycardia)

The normal resting heart rate for a cat typically falls within the range of 140-220 bpm. If your cat’s heart rate exceeds this, it could be a sign that it is suffering from an overactive thyroid. This is caused by excess thyroid hormone stimulating the heart.

This is not something that an owner is likely to notice. However, if a cat is brought to see the vet due to other symptoms, tachycardia could be detected during an examination. You may be able to notice the chest beating rapidly if you regularly cuddle your cat.

#11. Heart Murmur or Abnormal Heart Rhythm

When a vet checks your cat with a stethoscope, they may notice this problem with the animal’s heart. In itself, this is not a symptom of hyperthyroidism - it actually indicates secondary heart disease.

However, untreated or undetected hyperthyroidism can cause thickening of the heart muscle. If this goes unnoticed, it increases cardiac risk for the cat. This is one of the reasons it is important not to ignore other potential symptoms of hyperthyroidism.

#12. High Blood Pressure (Hypertension)

It is fairly common for a cat to develop high blood pressure if it has hyperthyroidism. This is something that isn’t likely to be noticed in itself, but a vet might discover it if you take your cat tobe examined after noticing another symptom.

Why High Blood Pressure Is Dangerous

Blood pressure measurement is recommended in older cats during routine check-ups because there are many conditions that can cause it. If a cat has high blood pressure and it is left unchecked, it can lead to major problems like:

• Sudden blindness (retinal detachment)

• Neurological issues

• Kidney damage

#13. Behavioural Changes

You know your cat well, especially if it has been with you for many years. They all have different personalities, and a sudden change in personality should not be ignored. A cat with hyperthyroidism could display new behaviours like:

• Irritability

• Aggression

Anxiety

Less tolerance of handling

Signs of being confused or distressed

These changes in behaviour are usually caused by overstimulation of the nervous system. If you observe anything like this, get your cat seen by a vet.

#14. Muscle Weakness

This is a less common symptom of hyperthyroidism - one that is more likely to occur later in the condition. It happens as a result of muscle wasting due to protein breakdown in the body as the metabolism is working overtime.

You may notice:

• Hind limb weakness

• Difficulty jumping

• Tremors (very rate, but possible)

In severe cases, you may even notice that the cat’s neck muscles become weak.

#15. Heat Intolerance

We all know cats - they love to be warm. On a hot day, they will often lie down in direct sunlight and allow themselves to bake in the sun. But an overactive thyroid can actually cause behaviour that suggests they dislike the heat.

Look out for behaviours like:

• Avoiding warm laps or beds

• Seeking shade and cooler areas

This happens because the increased metabolism is causing the cat to feel too hot.

Advanced Symptoms of Untreated Hyperthyroidism

If hyperthyroidism in cats goes untreated for a long time, the animal can develop more severe symptoms. This is why it is important that you don’t ignore any of the symptoms listed above - get your cat examined by a vet if you are concerned.

An untreated overactive thyroid in a cat can lead to the following:

• Severe weight loss (cachexia)

• Laboured breathing

• Heart failure

• Collapse

• Blindness due to hypertension

• Severe kidney dysfunction

If you have noticed any of the potential symptoms of hyperthyroidism in your cat, we strongly urge you to make an appointment with your vet as soon as possible.

When Should You Contact a Vet?

If your cat is aged 7 or older, any of the following symptoms should be enough to prompt you to make an appointment:

• Unexplained weight loss

• Increased appetite

• Drinking more

They could be symptoms of various things, including hyperthyroidism, so it is strongly recommended to get them examined.

You will need to call for an urgent appointment if you observe:

• Sudden blindness

• Collapse

• Extreme lethargy

Annual health checks are important for senior cats so that any developing problems can be caught early. When cats are over 10, blood screening is recommended in case there are any silent conditions bubbling beneath the surface.

How Vets Diagnose Hyperthyroidism in Cats

The first step of diagnosing an overactive thyroid is a physical examination. This will include weighing the cat, feeling its glands, listening to its heartbeat and asking the owner some questions.

If hyperthyroidism is suspected, the physical examination could be followed by:

• Palpation of an enlarged thyroid gland

• A blood test measuring total T4 hormone

• Blood pressure check

• Kidney function testing

Treating hyperthyroidism can sometimes reveal underlying chronic kidney disease. Both conditions are important to treat early to improve outcomes, which is why you must never ignore symptoms or changes in behaviour.

Final Thoughts

The most common early symptoms of hyperthyroidism in cats are weight loss, increased appetite, drinking more, urinating more and hyperactivity. If you notice these things in a cat aged 7+, it is strongly recommended that you book an appointment with your vet.

Hyperthyroidism is a very manageable condition if it is caught early. It can lead to serious complications if left untreated, which is why we emphasise the importance of seeing your vet.

Frequently Asked Questions About Symptoms For Hyperthyroidism in Cats

At what age do symptoms usually appear?

An overactive thyroid is most common in cats aged 10 years or older. It is very rare in cats under 7 years, but that doesn’t mean it is impossible.

Can symptoms come on suddenly?

While some of the more advanced symptoms may appear to come on suddenly, that will be after a long period of more mild symptoms. These are usually gradual, and behaviour changes tend to be the first thing owners notice.

Can hyperthyroidism be mistaken for kidney disease?

Yes, it is possible. Both can cause weight loss and thirst changes. The blood tests differentiate between the two, so if one is not present then a test for the other will usually be undertaken.