Feline Hyperthyroidism Explained: Causes and Treatment

This article takes a look at a common endocrine disorder seen in middle-aged and older cats; hyperthyroidism. We’ll explore the potential causes, clinical signs, diagnostics and various treatment options.

Thankfully, the prognosis for those who are being treated is very good, and signs will be generally be well managed. Patients can go on to live long and happy lives.

What is Hyperthyroidism in Cats?

Hyperthyroidism in cats is a hormonal disorder. When an excess of thyroid hormone is being produced, the cat’s metabolism goes into overdrive. In essence, the body is running at ‘high speed’ and suffers as a consequence.

An overactive thyroid gland can lead to a wide range of symptoms that can sometimes be mistaken for signs of getting older or other medical issues such as diabetes or kidney failure.

How Common is It in Cats?

With up to 10% of cats over the age of nine affected, this is a disease that many owners will have personal experience with. For veterinary clinics, many of their regular patients will be under treatment for thyroid disease.

Though frequently diagnosed in clinical practice, it is likely that many additional cats are living with the condition without their owner realising it.

It would not be uncommon for an elderly cat to be brought in for euthanasia, who has likely been dealing with an overactive thyroid for many years, but the owner was unaware they had a medical issue that needed treatment.

Which Cats Are Most at Risk?

It is a condition that we tend to see in those cats in the second half of their life. Hyperthyroidism is rarely seen in cats under 7 years old.

The most common breeds (Domestic Short and Longhairs) seem to be the most commonly affected, and there is some evidence that oriental breeds may have a degree of protection against developing an overactive thyroid.

Indoor cats and those fed on tinned or canned diets are thought to be at greater risk than their peers. Also, as chemicals and toxins in the home can settle on the fur, those who groom a lot may be more likely to develop hyperthyroidism.

What Causes Cat Hyperthyroidism?

The thyroid glands, which are located in the neck, can make too much thyroid hormone. When this occurs, the cat is known to suffer from hyperthyroidism.

Thyroid Adenoma

By far, the most common cause of this disease is a benign tumour of the thyroid gland called an adenoma. The tumour can affect one or both thyroid glands. These masses grow slowly over time.

Thyroid Carcinoma

For a small number of cats, the overgrowth within the thyroid gland will be a cancerous carcinoma. This is the case for less than 1 in 20 patients. This sort of cancer tends to grow rapidly and can spread around the body.

Environmental Factors and Chemical Exposure

Lots of studies have determined exactly what causes hyperthyroidism in our feline population, and some links exist. Regarding thyroid adenoma, there have been links between this tumour type and cats who live indoors and are fed canned diets.

Fire retardant chemicals (PBDEs), pesticides, and herbicides could all play a role.

What Are the Symptoms of Feline Hyperthyroidism?

The thyroid gland affects the whole body, which is why a cat with this condition will exhibit a vast range of symptoms.

Early and Subtle Signs to Watch for

One of the earliest symptoms owners seem to notice would be an increased appetite and personality change, with their kitties suddenly having more energy.

Advanced or Severe Symptoms

As this condition progresses, signs can become a lot more concerning. Without treatment, cats quickly become very skinny, develop a scruffy coat and may even have swelling in their neck.

Let’s take a closer look at what to be on the lookout for:

Behavioral Symptoms

Cats are often seen as skittish and hyperactive. They may want more fuss, become uncharacteristically aggressive, or call for more attention. This behaviour is sometimes confused with feline dementia. Indeed, some patients will be affected by both conditions.

You might also notice that your cat is restless and begins to pace and move more. Due to joint discomfort, this is also a behaviour we see in those with osteoarthritis. As both hyperthyroidism and joint disease are regularly seen in the senior population, both must be screened for.

Physical Symptoms

Some of the more common physical symptoms of this disease would include:

• Weight loss and muscle wasting
• A rapid heartbeat
• Diarrhoea (Cats)
• Vomiting
• Excessive hunger
• Increased thirst and urination
• An enlarged thyroid gland (goitre)
• A greasy and matted coat

It is worth mentioning that some cats with an overactive thyroid present differently. They may be lethargic and sluggish. This occurs in about 10% of cats, and it is known as ‘apathetic hyperthyroidism’.

If your cat displays any ‘red flag’ symptoms, such as open-mouth breathing or trouble walking, it is important that you contact an emergency vet right away.

How is Hyperthyroidism in Cats Diagnosed?

Veterinary Examination

During the consultation, your vet will ask you about your cat’s recent history and the signs that are concerning you. They’ll also weigh your cat and assess for any recent weight loss.

The physical exam should include a full check over, and the vet will feel for any goitre and check for a rapid heart rate. Your vet will also be assessing for any other possible causes of the signs, such as an abdominal mass, shrunken kidneys or thickened bladder wall.

Specific Tests for Diagnosis

If your vet wants to rule in or out feline hyperthyroidism, they’ll order some tests. Generally, this condition is diagnosed with blood tests. T3, T4, and free T4 may be measured. TSH is usually not measured, as hypothyroidism is very rare, and TSH levels are usually going to be very low.

In addition to ordering a thyroid profile to assess your cat’s thyroid hormone levels, your vet will want to review its haematology and biochemistry. This provides an overall picture of your cat’s health and organ function.

Additional tests can include a urine analysis, blood pressure measurement, and possibly abdominal imaging, depending on the symptoms your cat is displaying.

Blood tests tend to be very accurate at diagnosing the disease, but sometimes yield a ‘false negative’ result. This is when the cat is hyperthyroid but its thyroid hormone is suppressed to within the normal range due to other medical issues. For these cats, imaging and scintigraphy can be useful diagnostic tools.

Additional Tests to Check Heart and Kidney Health

Blood and urine tests at the time of diagnosis are especially important, as we know that untreated hyperthyroidism can have an effect on the kidney enzymes (artificially improving the levels), which will need to be closely monitored.

Many cats will also have some degree of chronic kidney disease, and your vet will want to know if this is the case before therapy begins. In addition to measuring urea and creatinine levels, SDMA should be checked whenever possible.

Those with an overactive thyroid may have hypertrophic cardiomyopathy, so a chest X-ray and echocardiogram may be ordered by your vet.

What Are the Best Treatment Options for Feline Hyperthyroidism?

Thankfully, this condition has many treatment options, and most owners can choose the therapy that best suits their budget and lifestyle.

Medication

Most cats are managed with medicine, at least initially. This can be oral medication, such as Methimazole or Thiamazole (tablets or syrup), or a gel that goes on the ear (Methimazole). Cats on the medicine require routine monitoring, and it can take a few months to find the optimal dose initially.

Dietary Therapy

There are prescription diets available with reduced iodine levels. If the cat eats this food alone, this alone can manage the condition for some patients. However, this is generally not a suitable option for those who live with other pets or who may eat when outdoors.

Radioactive Iodine Therapy (I-131)

This is a safe and effective therapy, but it is only offered at specialised centres such as universities and referral hospitals. This is a non-invasive and painless solution. The downside is that it is a one-off large expense (£3,000-4,000), and the cat needs to be isolated for a few days to weeks after therapy.

Surgical Removal of the Thyroid (Thyroidectomy)

This surgery aims to remove the tumour and leave healthy thyroid tissue behind. As with any surgery and general anaesthetic, risks will be involved, and not every senior patient would be a good candidate.

Pros and Cons of Each Hyperthyroidism Treatment

One of the biggest decisions an owner will make is which treatment option to pursue. While brilliant that so many exist, it can complicate matters.

Medical Management

Positives

• Affordable
• Non-invasive
• Reversible (if kidney disease is a significant issue once therapy begins)

Negatives

• Need for the owner to medicate daily
• Regular blood tests are a requirement
• Potential side effects (such as stomach upset)

Radioactive Iodine

Positives

• The Gold Standard
• One-off therapy
• High success rate (95-98%)

Negatives

• High initial cost
• Requires a specialist facility
• A short period of isolation afterwards

Surgery

Positives

• Can be curative
• One-off therapy

Negatives

• General anaesthetic risk
• Risk of damaging the parathyroid gland

Prescription Food

Positives

• Non-invasive
• Great option for cats who are hard to medicate

Negatives

• Not curative
• Cats cannot eat other food
• Not for those in multi-cat homes

Possible Complications of Untreated Hyperthyroidism

If hyperthyroidism is left unchecked, there will be a range of implications.

Heart Disease and High Blood Pressure

The stronger heartbeat and increased heart rate caused by the elevated thyroid hormone can enlarge the heart and result in irregular heartbeats.

These cats may also have high blood pressure, which can lead to conditions such as retinal detachment (causing sudden blindness) and bleeding in the brain.

Impact on Kidney Function

Increased blood flow to the kidneys can increase their workload and damage them over time.

Long-Term Management and Monitoring

Pet parents can help from home by ensuring their cats are getting any medicine they require and monitoring them for any changes to their symptoms.

How Often Should Blood Tests Be Repeated?

Initially, for those on medication, your vet will want to see them every 3 weeks until thyroid levels have stabilised.

They will then want to see them every 3-6 months, as the dose of their medicine may need to change, and their general condition and organ function should be monitored.

What Side Effects Should Owners Watch For?

Those with vomiting or diarrhoea would likely benefit from a bland senior diet, rehydration solutions, and supplements containing prebiotics and probiotics.

Lethargy and a reduced appetite may also be seen, especially in the early days of therapy.

When Should You Contact Your Vet?

If you are concerned for your cat or feel he is showing signs of hyperthyroidism, you should contact your vet for assessment and diagnosis.

Managing Feline Hyperthyroidism at Home

Pet parents can help from home by ensuring their cats are getting any medicine they require and monitoring them for any changes to their symptoms.

As cats can be ravenously hungry before their condition is controlled, it is important to keep them well away from unsafe foods or things like plants and medication.

Those with vomiting or diarrhoea would likely benefit from a bland senior diet, rehydration solutions, and supplements containing prebiotics and probiotics.

Get More Advice On Feline Hyperthyroidism

If you think your cat may have hyperthyroidism, or you have concerns about a cat that has already been diagnosed, please don’t hesitate to contact us to discuss your cat's condition.

Hyperthyroidism in Cats FAQs

Can Hyperthyroidism in Cats Be Cured?

Yes. Some treatment options are curative, meaning they can permanently resolve the condition. These include surgical removal of the affected thyroid tissue and radioactive iodine (I-131) therapy, which selectively destroys overactive thyroid cells. Other treatments, such as daily medication or prescription diets, manage the condition effectively but do not cure it.

How is Hyperthyroidism Diagnosed in Cats?

Veterinarians typically suspect hyperthyroidism based on a cat’s clinical signs, physical examination, and medical history. The diagnosis is usually confirmed with a blood test measuring thyroid hormone (T4) levels. In some cases, additional blood tests or imaging may be recommended to confirm the diagnosis or assess overall health.

What Are the First Signs of Hyperthyroidism in Cats?

Early signs can be subtle but often include increased appetite, weight loss despite eating well, hyperactivity or restlessness, and a poor or unkempt coat. Some cats may also vocalise more than usual or show changes in behaviour.

Can Hyperthyroidism Shorten a Cat’s Lifespan?

Yes, if left untreated, hyperthyroidism can lead to serious complications such as heart disease, high blood pressure, kidney problems, and muscle wasting, which can significantly shorten a cat’s lifespan. However, with appropriate treatment and monitoring, many cats go on to live several healthy years after diagnosis and maintain a good quality of life.