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Hyperthyroidism in Cats: Causes, Symptoms & Cure

You will almost certainly have heard the term ‘hyperthyroidism’ before. But what exactly is it? What causes it, and is it as common in cats as it is in people?

Can cats have hyperthyroidism? They can, just like we can. Hyperthyroidism is caused by overactive thyroid glands, a gland located in the throat which produces hormones that regulate bodily processes. Your cat has two of them, one located on the left and one on the right. They become overactive when they get bigger, typically because of tumors that appear in old age. When they’re bigger, they produce more hormones, which causes basal energy expenditure (metabolism) to increase. This causes weight loss, increased appetite, poor coat quality, restlessness, polydipsia and polyuria and tachycardia (increased heart rate). The problem can be fixed best with radioactive iodine injections, surgery, medicine or dietary changes. With appropriate medical care, hyperthyroidism does not affect quality of life or lifespan.

The guide below first looks at what causes hyperthyroidism in cats before looking at the key symptoms you’re likely to spot. It answers key questions like why an overactive thyroid has the knock-on effects that it does, and how long can cats live with hyperthyroidism. It will then recommend several courses of action to be decided on by you and your vet.

What Causes Hyperthyroidism in Cats?

Hyperthyroidism is reasonably common, particularly in middle-aged and elderly cats. It is caused by increased production of two thyroid hormones, known as T3 (triiodothyronine) and T4 (thyroxine). These hormones perform several key jobs, such as regulating weight and energy level, internal temperature, and the growth of things like skin, hair and claws. These are the same jobs that they perform in the human body.

When the thyroid glands are bigger than normal, they produces more of these hormones than normal too. There are several reasons why your cat’s thyroid glands might be bigger than normal, although the most common is a kind of non-cancerous tumor called an ‘adenoma’. It’s also possible for cancerous tumors to cause an enlarged thyroid gland.

It’s not clear what triggers the initial growth of these tumors and why they are common in middle-aged and older cats. There is no obvious link between hyperthyroidism and diet, exercise level or breed that adequately explains it. As such, you should be aware of the condition and its effects even if your cat is typically very healthy.

To be clear, hyperthyroidism is a separate condition to hypothyroidism. Hypothyroidism is the opposite, where the thyroid gland is under-active.

What Are Hormones and What Do They Do?

To understand why hyperthyroidism is a problem, it’s necessary to fully understand what hormones are and what they do. The MSD Manual states that ‘hormones are chemical substances that affect the activity of another part of the body (target site). In essence, hormones serve as messengers, controlling and coordinating activities throughout the body.’ They go on to say that

Upon reaching a target site, a hormone binds to a receptor, much like a key fits into a lock. Once the hormone locks into its receptor, it transmits a message that causes the target site to take a specific action. Hormone receptors may be within the nucleus or on the surface of the cell.

Ultimately, hormones control the function of entire organs, affecting such diverse processes as growth and development, reproduction, and sexual characteristics. Hormones also influence the way the body uses and stores energy and control the volume of fluid and the levels of salts and sugar (glucose) in the blood. Very small amounts of hormones can trigger very large responses in the body.

The body produces these hormones in small organs called glands. These glands are found throughout the body, each producing a different but no less important hormone than the others. This explains why an overactive thyroid gland has such wide-ranging effects, as we’ll see in a moment.

What Are The Symptoms of a Cat with Thyroid Problems?

Hyperthyroidism causes many different symptoms which appear throughout your cat’s body, not just around its neck. That’s because the two hormones that the thyroid gland produces have effects all over the body. The key symptoms you will see are weight loss, increased appetite and poor coat. Let’s take a look at these key symptoms in more depth.

Hyperthyroidism and Weight Loss in Cats

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Cats with hyperthyroidism will lose weight gradually until they are very thin.

Hyperthyroidism makes cats lose weight. This weight loss can get so bad that your cat will become very thin, so thin that its life is endangered.

The reason why hyperthyroidism causes weight loss is that the hormones the thyroid creates regulate the body’s metabolism. While the science of metabolism has long been misunderstood, it is a real thing, measured using a metric called ‘basal energy expenditure’. This is the amount of energy, measured in calories, that the body needs to survive over a given period of time.

Cats with thyroid issues have a higher basal energy expenditure than normal. This means that your cat burns energy more quickly than it can take it in. Over time, this deficit adds up, so that your cat loses weight.

If the condition is left untreated, your cat’s energy expenditure won’t balance out. It will continue to lose weight until it is dangerously thin.

Hyperthyroidism and Appetite in Cats

This decrease in weight is accompanied by an increase in appetite. Appetite is regulated by thyroid gland hormones, too. You may therefore notice your cat eating more frequent meals, eating larger meals each meal time, demanding more snacks than usual, stealing meals from other cats’ bowls, or getting food from other houses.

But even though your cat has an insatiable appetite, it still won’t eat enough to stop it from losing weight. Its increased appetite will slow down its weight loss, but it’s unlikely to stop it altogether.

Hyperthyroidism and Polyuria/Polydipsia in Cats

Hyperthyroidism affects the whole body. It particularly affects the kidneys and the heart. Increased levels of thyroid hormones affects the production of two proteins called aquaporin 1 and 2, which increases the frequency of urination. Increased urination is known as ‘polyuria’. Another reason why cats with hyperthyroidism urinate more is polydipsia, i.e. increased drinking. This goes hand in hand with your cat’s increased appetite. When your cat drinks more, it goes to the toilet more.

Besides that, cats get much of their water from their food, at least if they eat wet food. If your cat is eating more than normal, then it’s also taking in more water than normal through its food, and that water has to be excreted somehow.

Hyperthyroidism and Coat Quality in Cats

You may also notice that your cat’s coat changes. It will become less lustrous, more greasy, and more easily matted. This will occur even if your cat grooms itself as it normally does. Again, this is because of the knock-on effects that thyroid gland hormones have.

These are not the only symptoms your cat will experience, but they are the most common. Other symptoms include restlessness, defensiveness/aggression, vomiting, diarrhea, weakness and depression. These symptoms are all associated with general ill health.

Is Hyperthyroidism Fatal For Cats?

Hyperthyroidism can kill if it’s left untreated. That’s because the hormones that the thyroid gland produces affect so many different parts of the body.

Hyperthyroidism in Cats Life Expectancy

The life expectancy of your cat depends on how severe the problem is and when it occurs. Since the average age of cats that develop this condition is 13, most hyperthyroid cats are nearing the cat’s typical lifespan already. If you seek treatment for your cat as soon as possible, then it can live a happy and healthy life no shorter than that of any other cat. Most treated cats live another three to five years.

This also depends on whether the treatment is successful or not. There is no treatment for hyperthyroidism that is successful every time, and if the treatment isn’t a success, then your cat’s lifespan will be shorter. There’s also the chance that the treatment will have side effects, which applies to all forms of treatment.

Signs Your Cat Is Dying of Thyroid Disease

In late stage hyperthyroidism in cats, you can expect the symptoms described above to get worse. The effect of excess T3 and T4 hormones does not balance out over time, meaning that your cat will continue to lose weight. As such, the main sign that your cat’s thyroid disease could kill it is if it’s dangerously underweight.

Knock-On Effects of Hyperthyroidism

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Cats with hyperthyroidism may vomit. That’s because hyperthyroidism can damage the kidneys, meaning they don’t do their jobs properly, which in turn means that there’s a buildup of toxins in your cat’s bloodstream and digestive system.

The key reason why hyperthyroidism can prove fatal is that it affects the kidneys and heart. When the condition is allowed to continue without treatment, the problems it causes in these vital organs will kill your cat.

Even the earliest stages of hyperthyroid affect the heart. Elevated levels of thyroid hormone cause increased heart rate, known medically as tachycardia. Alongside this issue, each contraction of the heart’s muscle tissue is stronger than normal. These hormones gradually cause the left ventricle of the heart to thicken, which is one of the many root causes of heart failure. It’s for this reason that the vet may also prescribe heart medication alongside thyroid medication.

Excess thyroid hormones also cause hypertension, i.e. high blood pressure. This affects the heart, of course, but it also affects all the other organs in the body.

Can Cats Recover from Hyperthyroidism?

Cats can recover from thyroid issues so long as appropriate medical care is found. It’s therefore vital that you seek medical treatment for your cat as soon as possible.

How Do Vets Diagnose Hyperthyroidism in Cats?

The first step you should take when your cat is ill with any health condition is to talk to a vet. The vet is there to help, and can use special diagnostic techniques that they have used many times before to identify what’s wrong.

The vet will begin by manually checking your cat’s throat. They will palpate (feel) the area to see if the thyroid is enlarged. They will also check your cat’s body generally to see if other symptoms are present, such as matted coat, muscle weakness, and so on. If they suspect that your cat has a thyroid issue, then they will perform several other tests too, for example a blood chemistry panel: this is where a small amount of your cat’s blood is taken and checked. Cats with hyperthyroid issues have high levels of thyroid hormones in their bloodstream which can be picked up in tests. The vet may also a urine test for the same reason.

Beyond diagnostic checks for thyroid problems, the vet will also look at your cat’s general health. That’s because the hormones that the thyroid gland produces affect lots and lots of places around your cat’s body. They will check your cat’s kidneys and heart in particular, as these can be negatively affected. The vet may therefore also order an ultrasound of your cat’s heart to check its health.

Following a successful diagnosis, the vet will order one of four forms of treatment.


The vet can prescribe a kind of medication known as ‘anti-thyroid’ medication. These drugs counterbalance the fact that a large thyroid makes lots of hormones by encouraging it to make fewer. This is likely to be the first course of treatment that the vet offers, on account of anti-thyroid drugs being cheap and widely available. They serve to get the condition under immediate control, restoring some kind of balance to your cat’s regulatory systems (e.g. weight and appetite) so that it can recover.

There are several downsides to administering anti-thyroid medications, however. One is that they do nothing to combat the underlying problem of your cat’s enlarged thyroid. You will therefore have to give your cat these medications permanently, as should you ever stop, your cat’s thyroid will again begin producing too much T3 and T4 hormones. Another issue is that your cat may experience side effects, some of which are what it would experience with an overactive thyroid gland anyway. A cat on anti-thyroid medication may experience vomiting and lethargy, and may continue losing weight due to anorexia. Other side effects include anemia and fever.

If your vet prescribes anti-thyroid medication for your cat, follow their prescription. You are likely going to need to give your cat two doses per day, and do so for the rest of its life, if this is the treatment selected.

Radioactive Iodine Therapy

Radioactive iodine therapy is a highly specialized form of therapy that is very effective. It may not be available where you live, but if it is, your vet may recommend it.

Your cat will be injected with irradiated iodine, a chemical substance. This will be administered straight into the bloodstream. Because the thyroid gland needs iodine to produce the hormones it produces, it will be absorbed there. The fact that it’s radioactive means that it will destroy any abnormal thyroid tissue there, meaning that the thyroid can’t produce excess hormones any more. This procedure is therefore a kind of chemotherapy, i.e. a therapy where an irradiated chemical substance is used to kill tumerous tissues.

Because this treatment involves radioactive materials, it can’t be performed at your vet’s office. It’s only on offer from specialist centers that are licensed to administer radioactive treatments. Your cat will also have to stay at the facility for several days, because while the treatment won’t harm your cat, it will be radioactive for a brief period! It will be monitored until the level of radioactivity it produces is within a range that won’t affect human health.

This might sound scary, but it won’t hurt your cat—talk to your vet if you have concerns about this procedure. It almost always works and won’t affect your cat’s lifespan negatively. The only issue is that in rare cases, the destruction of thyroid tissue goes too far, and hypothyroidism is the result. This has its own negative effects like weight gain and poor coat.


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Surgery is an option, but not the best option available.

It’s also possible for a veterinary surgeon to remove your cat’s thyroid glands through a procedure known as a ‘surgical thyroidectomy’.

This is a simple surgery performed with the cat under general anesthetic. Despite being simple, though, this treatment method is rarely offered. That’s because it’s more invasive than radioactive iodine therapy. There’s also the chance that the surgeon will cause damage to other glands in the throat (the parathyroid glands) which like the thyroid help regulate the body’s many processes. If these are damaged, then surgery can do just as much harm as good.

Another problem with surgery is that it requires anesthetic. Anesthetic can kill a cat. This can happen because cats are so small that its more difficult to get the level of anesthetic right: too little and they are awake for the operation, too much and they might pass away. While veterinary anesthetists do a difficult job well, accidents can happen, particularly to older cats. You should therefore double check with your vet that surgery is the correct choice, particularly if alternatives are available.

Dietary Therapy

If you have hyperthyroid too, or you know somebody that does, you may have heard that dietary changes can help. Unfortunately, this isn’t a straightforward fix.

There are studies that suggest that a low-iodine diet can fix some of the issues related to hyperthyroidism in cats. This paper in the Journal of Veterinary Internal Medicine looked at the topic in 2015. The authors state that research into dietary therapy for hyperthyroidism in cats is lacking, although early indications point towards a low-iodine diet helping. They therefore set out to check whether that’s the case with a larger study than was ever done before.

The food they used was a commercially available ‘complete’ food which is low in iodine. This diet was fed to a selected group of cats for six months. The authors found that

[t]his retrospective study documents that exclusive feeding of a restricted‐iodine diet is effective in normalizing and maintaining normal serum TT4 concentrations in the majority of cats with spontaneous hyperthyroidism by 61–180 days after starting the diet; however, in these cats as a group, there was no improvement in objective clinical signs such as body weight or tachycardia. […] Normalization of serum TT4 concentration by feeding an iodine‐restricted diet did not lead to significant improvement in objective physical examination variables; median heart rate did not decrease and median body weight did not increase.

[…] As weight loss is thought to occur in hyperthyroidism because of increased basal metabolic rate from excess thyroid hormone circulation, normalization of TT4 concentration in the cats of this study should have led to an increase in body weight. There are a couple potential explanations for this lack of change. The first is that despite normalization of serum TT4 concentrations, some of the cats in this study might have still been physiologically hyperthyroid. […] Another possibility is that the many concurrent illnesses present in this population of geriatric cats might have also influenced their body weight. This theory is supported by the fact that although many cats did not gain weight, the body weights stabilized and did not continue to decrease after starting the diet. [Emphasis added]

In other words, a low-iodine diet did stop the cats from overproducing thyroid hormones. But what it didn’t do was fix any of the symptoms associated with hyperthyroidism—which is an unusual result. This suggests that further studies are needed to establish what a low-iodine diet does and doesn’t achieve for cats with hyperthyroidism.

If you have no other option, you should also look to feed your cat a high-calorie diet. This will stave off your cat’s weight loss, or in combination with other treatment methods, help it gain its weight back. The best way to do this is by feeding high-calorie treats, particularly any that contain lots of protein and fat.

When to Put a Cat with Hyperthyroidism to Sleep

There is no need to put your cat to sleep if it has hyperthyroidism. The treatment options above are widely available and are effective. They extend your cat’s lifespan to a normal range, but perhaps more importantly, stop your cat from experiencing suffering and discomfort. Cats with this condition can make full recoveries, so talk to your vet about your options.

There are only two exceptions. One is if the disease has almost fully run its course, and your cat is very seriously underweight and ill. If that’s the case, then it may have a more difficult road to recovery. The other exception is if your cat is very old, which can complicate recovery from medical interventions like surgery. Rather than being alone in your worries, talk to your vet about the likelihood that your cat will make a full recovery.

Are Cats With Hyperthyroidism in Pain?

If you successfully treat the condition, then no, your cat will not be in pain. Even treatments that manage the condition rather than fix its underlying cause (like medication) will stop your cat’s suffering.

But if you leave the condition entirely untreated, then the knock-on effects of your cat’s enlarged thyroid will cause it pain. It may experience heart pain due to tachycardia and hardening of the heart’s muscle tissue, for example. It may also feel sick in a general sense, which isn’t painful per se, but is an unpleasant experience for your cat to go through. The same applies to your cat’s insatiable hunger and its progressive weight loss: while these things aren’t painful, they are most definitely unpleasant.

Again, though, these things can be fixed with appropriate treatment. You therefore don’t need to worry about your cat ‘suffering’ if you manage or cure its health issue.