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 Feline Hyperthyroidism?

Feline hyperthyroidism 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.

Causes of Feline 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.

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.

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

How to Diagnose Feline Hyperthyroidism

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

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 in diagnosing the disease, but sometimes, there will be 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.

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 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.

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.

For those with vomiting or diarrhoea, they 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.