Hyperthyroidism in Cats: Symptoms, Causes, and How to Treat It

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As cats age, they're at an increased risk of developing certain health conditions. One of the more common conditions is hyperthyroidism. This condition can affect your cat's metabolism, leading to symptoms like weight loss and increased thirst.

Fortunately, with proper treatment, hyperthyroidism can be managed in many cats. Learn about treatment options for hyperthyroidism in cats, along with the symptoms to look out for and possible causes.

What causes hyperthyroidism in cats?

Hyperthyroidism is caused by an overproduction of thyroid hormone. "The thyroid gland is a butterfly-shaped organ located in your cat's neck right over the trachea," Sara Ochoa, DVM, a veterinarian with Whitehouse Veterinary Hospital, told GoodRx Health.

When the thyroid becomes enlarged, it can lead to hyperthyroidism. Almost 99% of the time, a benign (noncancerous) tumor, called an adenoma, is the reason behind the enlarged gland. In rare cases, a malignant tumor, known as an adenocarcinoma, enlarges the thyroid gland.

Are certain cats more prone to hyperthyroidism?

Older cats seem most prone to hyperthyroidism. The average age of cats with hyperthyroidism is 13 years old. Hyperthyroidism rarely occurs in younger cats. Only approximately 5% of cats with hyperthyroidism are younger than 10 years old.

Some studies suggest that the following factors may increase a cat's risk of hyperthyroidism:

• Eating canned food, especially canned food that contains fish, liver, or giblets

• Having contact with soy isoflavones, polyphenols, or phthalates found in some cat foods and other products in the home

• Living primarily indoors

• Being around flea sprays, fertilizers, pesticides, or insecticides

• Having contact with polychlorinated biphenyls found in pet foods, indoor dust, and household plastics

Signs and symptoms of hyperthyroidism in cats

Hyperthyroidism increases a cat's metabolic rate, which can result in many symptoms. The most common symptom is losing weight despite eating more than usual.

Other common symptoms of hyperthyroidism include:

• Increased thirst

• Increased urination

• Vomiting

• Diarrhea

Cats can also experience heart-related symptoms such as:

• Shortness of breath

• Increased heart rate

• Heart murmurs

• An enlarged heart

• Congestive heart failure

• High blood pressure, which can lead to blindness if not treated

Diagnosing hyperthyroidism in cats

If you notice your cat having symptoms of hyperthyroidism, talk to your veterinarian. They will examine your cat to determine if hyperthyroidism or another condition is at fault.

"Many times, if your vet thinks that your cat has elevated thyroid levels, they will be able to feel this gland enlarged in their neck," Ochoa sayid. "[Your vet] will palpate their thyroid gland to see if it is enlarged or if there is a mass growing in this area."

Your vet will also run blood work on your cat to check for thyroid problems. They may also check for other issues that are common in older cats, such as diabetes and kidney disease.

Treatment for hyperthyroidism in cats

Ochoa explained that there are four potential treatment options for cats with hypothyroidism. Your vet can help you determine which options are best for your cat.

1. I-131 radioactive iodine therapy

"Radioactive iodine treatment is the gold-standard treatment for cats with hyperthyroidism," Ochoa said, adding that it's also the most expensive treatment. "Depending on where you live, this procedure can cost a few thousand dollars."

Cats undergoing iodine-131 (I-131) therapy, or radioactive iodine therapy, receive an injection of radiation. The radiation travels into the thyroid and destroys abnormal thyroid tissue. It doesn't affect normal thyroid tissue or other organs around the thyroid gland.

"Most cats who are treated with radioactive iodine will have normal thyroid levels about 2 weeks after treatment," Ochoa said. "This treatment usually cures hyperthyroidism, and it doesn't have any serious side effects. Cats don't have to be sedated to receive the treatment."

The radiation can cause side effects for people who come into contact with a cat who's received 1-31 therapy. So, as Ochoa explained, most treatment facilities keep cats in isolation and require that they stay for at least a week before going home.

2. Methimazole

Methimazole is a medication that's used to treat hyperthyroidism in cats. Cats usually need to take it twice a day, with doses spread about 12 hours apart.

"This would be lifelong medication," Ochoa said. "This is usually the cheaper [treatment] option, as thyroid medication is very inexpensive."

Some cats taking methimazole experience side effects such as:

• Lethargy

• Loss of appetite

• Vomiting

• Facial itching

• Liver disease (in rare cases)

• Bone marrow changes (in rare cases)

If your cat experiences side effects while taking methimazole, talk to your vet about reducing their dosage or changing their treatment plan.

3. Surgery

Ochoa noted that a vet can remove a cat's thyroid gland if tests indicate that this is the best treatment option. Vets often only remove one gland, leaving the other to produce thyroid hormone for the cat's body. But there can still be side effects.

Cats who have thyroid surgery may develop hypothyroidism, a condition in which the thyroid gland doesn't produce enough hormone. Cats may also have issues with regulating calcium in their bodies. The thyroid gland is located close to the parathyroid gland, which is responsible for regulating calcium.

Ochoa explained that thyroid surgery on cats is often performed at a specialty clinic and can be very costly. And the procedure is not without risks.

"Since thyroid issues are usually noticed in older cats, there is an additional risk from your cat undergoing anesthesia and surgery at this age," Ochoa said.

4. Low-iodine diet

According to Ochoa, some recent studies indicate that you can help manage a cat's hyperthyroidism by feeding them a diet low in iodine. "This is very beneficial for cats who do not tolerate any of the other treatments," she said.

Ochoa recommended consulting with your veterinarian to determine what diet will be best for your cat.

"Iodine should be limited but not restricted from your cat, as they do need some iodine to live a long and healthy life," she said. "Some pet food manufacturers make a diet specifically for cats with thyroid issues."

What is the life expectancy of a cat with hyperthyroidism?

Cats whose hyperthyroidism is well managed can live long and healthy lives, Ochoa explained. But getting the proper treatment is crucial.

"An untreated cat [with hyperthyroidism] will have an elevated heart rate, which can quickly lead to other heart issues," Ochoa said. "This will, in turn, put stress on the rest of the organs in your cat's body, causing them to begin having problems functioning correctly."

Your vet will go over your cat's treatment options and what you can expect. They will likely also recommend follow-up thyroid checks for your cat.

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