Hyperthyroidism in cats is a common condition in middle-aged to older cats. The clinical signs of feline hyperthyroidism include increased appetite with weight loss. Left untreated, cats develop high blood pressure, kidney disease and heart problems.

With treatment, most cats with hyperthyroidism can lead a normal life for months and years to come.

There are lots of treatment options for feline hyperthyroidism. This includes special food, medication such as methimazole, surgical removal, or radioactive iodine treatment. Your veterinarian is best placed to discuss which treatment is best suited to your cat.

Enlarged Thyroid: What is Hyperthyroidism in Cats?

Hyperthyroidism in cats is what’s caused an endocrine disorder. ‘Endocrine’ refers to the hormones naturally produced by the body. In this case, the glands are the thyroid glands and the hormone is thyroxine.

The exact cause of thyroid disease is not known. However, the effect is well known, an overproduction of thyroid hormone. When the thyroid gland produces too much thyroid hormone, this pushes up the metabolic rate. This is the rate at which the body burns calories.

Most hyperthyroid cats have a condition where the gland is overactive and simply produces too much thyroid hormone. However, a small percentage of cases can be due to a more serious cancer or tumor affecting the glands.

Cat Thyroid: What Does Thyroid Do?

Cats have two thyroid glands: One on the left and right-hand side of the neck. Each thyroid gland produces thyroid hormone, which regulates the metabolic rate. When the thyroid hormone levels are too high, this is what’s known as hyperthyroidism.

Hyperthyroidism vs Hypothyroidism: There’s a Difference!

The prefix ‘hyper’ means ‘over’ or ‘too much’ of something. Another example is hyperthermia when a body is too hot.

The prefix ‘hypo’ means ‘under’ or ‘not enough’. An example of this is hypothermia when the body is too cold.

Thus hyperthyroidism and hypothyroidism are two very different conditions. Whereas hyperthyroidism is common in cats but rare in dogs, the opposite is true for hypothyroidism.

An underproduction of thyroid hormone means the body is sluggish and the patient lacks energy. Typically, a cat with hypothyroidism would gain lots of weight despite not eating much. In contrast, hyperthyroid cats have weight loss despite a big appetite.

Other Thyroid Problems in Cats

Anything that prompts the thyroid glands to produce too much thyroid hormone, will cause symptoms of hyperthyroidism. Commonly this is due to ‘hyperplasia’ or over-enthusiastic glandular tissue.

Alternatively, a relatively harmless cystic type tumor called an adenoma can affect the thyroid glands. This is not too much of a worry, because the main symptoms are those of hyperthyroidism.

However, a small percentage of cases are caused by a more serious type of tumor. A cancer of the thyroid gland called a carcinoma is more aggressive.

The Hyperthryoid Cat: What Causes a Swollen Thyroid

What is it that triggers the thyroid to become overactive?

This is an intriguing question to which the answer is largely just a theory. Hyperthyroidism was unknown before 1979. Researchers are suspicious that a man-made chemical may be responsible.

For a long time, it was thought that chemicals in cat food, specifically canned food were to blame. However, recent research now points towards the flame-retardant chemicals used in soft furnishings.

Whatever the cause, we know that certain cats are at increased risk. These are older cats that eat wet food and use a litter box.

Feline Hyperthyroidism: Symptoms of Hyperthyroidism in Cats

The symptoms of thyroid disease are those linked to a high metabolic rate. Typically, this makes the cat lively and skittish, almost kitten-like at times. These cats are hungry, to the point that they nag and pester for food.

But despite having a good appetite, feline hyperthyroidism causes the cat to burn calories too quickly and so the patient loses weight.

High levels of thyroid hormone also over-stimulate the gut. This leads to symptoms of hyperthyroidism such as sickness and diarrhea.

Ultimately, overactive thyroid tissue pushing out lots of, causes high blood pressure (or hypertension). This places the cat at greater risk of having a stroke or going blind.

Long-term hypertension then causes secondary damage to other organs. Typically it strains the kidneys and leads to chronic kidney disease. Also, the heart is forced to pump too quickly, and that racing heart rate leads to damage to the muscle of the heart and cardiac disease.

Diagnosis: Here’s How Your Vet Will know if Your Kitty has an Overactive Thyroid!

The veterinarian will have a suspicion based on the signs of hyperthyroidism in cats. An active, thin cat that has a racing heart will immediately flag up hyperthyroidism. The vet will carefully palpate the cat’s neck, feeling for enlarged thyroid glands.

A definitive diagnosis of hyperthyroidism is made with blood tests. This measures the levels of thyroid hormone in the bloodstream.

Further tests may be necessary to assess renal function and the state of the heart. This may mean further blood tests and a heart scan. It is not possible to decide whether the thyroid gland is merely overactive or has cancerous change, unless the tissue is biopsied.

Treating Cat Hyperthyroidism: Methimazole for Cats

There are several options for the treatment of hyperthyroidism. The choice of treatment depends on many factors. For example, a cat that is difficult to pill would do better with surgery or radioactive iodine treatment. Whereas a lone cat has the option of eating a special diet that is low in iodine.

With the exception of eating a special food, the other treatment options all require a period of stabilization first. This means using anti-thyroid drugs to normalize the levels of thyroid hormone and give the heart a rest and assess any long-term kidney damage.

The first treatment to be developed is still the most widely used option. This uses the anti-thyroid drug methimazole to reduce the levels of circulating thyroid hormone. Originally methimazole was only available as a pill. But in recent years other formulations have become available, including a liquid, an oral paste, and a gel that is applied to the ear.

Methimazole Side Effects

Methimazole is considered a safe medication, however, any drug has side effects and this is no different.

Some cats seem particularly sensitive to methimazole and develop severe inflammation and soreness of the skin on the face and ears. If this happens the medication should be stopped, and a short course of anti-inflammatory and antibiotic may be necessary.

A more complex side effect of methimazole treatment is that it can unmask hidden kidney disease. This is the result of high blood pressure coming down to normal, which means reduced blood flow to the kidneys. This then shows up as a worsening of renal disease.

This is a delicate situation because treatment of one problem means the worsening of another. The veterinarian then faces a dilemma over which condition is the most urgent to treat.

A Hyperthyroidism Diet to Help Your Feline Friend

A neat treatment option is to feed hyperthyroid cats a low iodine diet.

The body needs iodine to manufacture thyroid hormone. When fed a restricted amount of iodine the overactive glands are ‘starved’ of raw ingredient which normalizes thyroid levels.

Sadly, this treatment does have its downsides. This diet is only suitable for hyperthyroid cats. In a multicat household, this means feeding the other cats separately.

Also, many common foods or medications contain iodine. If the cat gets a source of iodine from somewhere else, such as stealing a mouthful of regular cat food then the good work of the special diet is undone.

Radioactive Iodine Therapy

The gold standard treatment is radioactive iodine therapy. This involves the cat being given an injection of a carefully controlled level of radioactive iodine. The overactive thyroid glands then soak up the radioactive gland, which selectively destroys the thyroid tissue.

The great news with radioactive iodine treatment is that it can blitz overactive thyroid tissue in hidden parts of the body. Also, the vast majority of cats then go back to having normal thyroid function. However, a very few cats go on to develop underactive thyroid glands as a result of this treatment.

Not all cats are suited for radioactive iodine therapy. It does require the cat to be hospitalized for a few weeks in a special facility. Cats that are overly anxious or aggressive may not cope well with this.

Hyperthyroidism in Cats Life Expectancy

Untreated, a hyperthyroid cat has a shortened life expectancy. This is due to complications such as high blood pressure, kidney disease, and heart failure. How quickly the cat deteriorates depends on lots of factors and varies from cat to cat.

Your veterinarian is best placed to guide you on life expectancy. But on average, cats live for months, or even a year or so, after diagnosis.

The outlook with treatment is much better. Many cats now survive for years after diagnosis, provided they receive treatment early before the heart is badly damaged.

Prognosis: Living with Hyperthyroidism in Cats

Hyperthyroidism is a treatable condition. When the problem is caught early, there is every chance the cat will lead a long and active life for many years to come. Key to this is finding the treatment that is right for your cat, and discussion with your veterinarian is essential.

Common Questions on Hyperthyroidism in Cats

What Causes Hyperthyroidism in animals?

Are there hyperthyroidism symptoms in cats?

Is there a treatment?