Hyperthyroidism in Cats: Causes, Symptoms, and Treatment

Hyperthyroidism (sometimes called thyrotoxicosis) is the term used to describe the condition in which cats have an excess in thyroid hormone production due to an enlarged thyroid gland. Your cat may acquire associated disorders and experience a variety of symptoms if their thyroid hormone production is out of balance. Positive therapeutic effects are observed, nevertheless. Learn about the causes, signs, and treatments of feline hyperthyroidism!

Hyperthyroidism in Cats: Causes, Symptoms, and Treatment
What Causes Hyperthyroidism in Cats?
Benign Tumor

The most common cause of hyperthyroidism is an adenoma, which is a benign tumour.


It’s interesting to note that some canned wet feeds with a fish flavour have been connected to cats developing hyperthyroidism.


Cats that have hyperthyroidism frequently do not have a genetic susceptibility to the disorder. Hyperthyroidism typically affects middle-aged and older cats. Cats are diagnosed with this condition on average at the age of 13.


Your home’s furniture might contain PBDEs, or flame-retardant compounds. These substances travel throughout your house and become airborne. They have been connected to cats developing hyperthyroidism.

Thyroid Cancer

Rarely does thyroid malignancy lead to the development of hyperthyroidism. However, it has happened.

What signs of hyperthyroidism do cats exhibit?

The symptoms of hyperthyroidism can take many different forms. See your veterinarian to identify the underlying source of any symptoms you observe in your cat, whether it’s hyperthyroidism or something else.

Both diarrhoea and vomiting

A cat that has recently tried a new food or gets into something it shouldn’t can throw up or have diarrhoea. Watch out for your feline companion if it’s a standalone incident with a clear reason. But it’s a good idea to make an appointment for a veterinarian if these symptoms are accompanied by others.

Changes in Behavior

It’s possible that your normally meticulous groomer has suddenly become dishevelled. It could appear as though your cat is ill. Additionally, you could observe hyperactivity in them—not the zoomies sort, but a restlessness that prevents them from settling in. You can also start to notice unusually aggressive behaviour.

Changes in Breathing

Another symptom of hyperthyroidism in cats is dyspnea, or trouble breathing. Additionally, tachypnea—rapid breathing—may be observed.

Enhanced Cravings and Reduction in Weight

Your cat might act hungrier than normal. However, even though their appetite is more ferocious, they could also lose weight.

Rapid Heart Rate

Your cat can have irregular heartbeats or a fast heart rate.

Changes to Nail Appearance

Hyperthyroidism may also cause your cat’s nails to thicken.

How Is Hyperthyroidism in Cats Diagnosed?

Your veterinarian searches for common symptoms and measures the amount of thyroid hormone in your cat’s blood to diagnose hyperthyroidism. Since hyperthyroidism symptoms can be confused with those of other illnesses, your veterinarian will do further testing in order to confirm a reliable diagnosis before starting therapy.

How Is Hyperthyroidism in Cats Treated?

A-Z-Animals was informed by Dr. Colleen Wallace, a cosy cat veterinary hospital in Raleigh, NC, that there are two primary approaches to treating hyperthyroidism in cats. The first and most widely used approach is radioactive iodine therapy, which is commonly used to treat hyperthyroidism. It entails boarding the cat at a specialised facility while it receives treatment. A painless injection of radioactive iodine is administered, and when it accumulates in the thyroid tissue, it destroys the malignant cells. After that, the cat remains at the facility for around a week or until it is no longer radioactive. In 90% of the cats, the illness is completely and permanently cured with this technique.

She continued, “The second approach involves administering a drug called methimazole, which binds excess thyroid hormone in the bloodstream and prevents the cells from using it. This can temporarily regulate thyroid hormone, but it has no effect on the thyroid tumour itself, thus the condition will continue to worsen.

As this occurs, you will need to occasionally do lab work to track the tumor’s growth and adjust the dosage of medicine as necessary to stay up with the tumor’s growth. This approach will only be effective for two to three years, and as a result, it should only be used with other cats because there is a maximum amount of medication that a cat can handle. While Hill’s Science Diet y/d diet is used by some vets, organ damage persists and thyroid function is not properly regulated. It is not something I advise my patients to do.