Hyperthyroidism in cats is a common disorder in older cats where their bodies make more hormones than needed. Recognize the signs and symptoms so you can find a treatment plan to help your cat live a long, happy life in spite of their condition.

By Sarah Mouton Dowdy
August 24, 2020
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Hyperthyroidism is a common endocrine disorder in cats in which their bodies start making more thyroid hormone than they need. The condition tends to affect older cats and can lead to serious secondary health issues, including heart failure. But cats that receive therapy can have great outcomes—especially if the signs of hyperthyroidism are caught early. 

How Do Cats Develop Hyperthyroidism?

“Hyperthyroidism is a condition in which the thyroid gland, which is located in the neck, produces too much thyroid hormone,” Leah Cohn, DVM, Phd, DACVIM, an internal medicine specialist and professor at the University of Missouri College of Veterinary Medicine in Columbia, Mo., explains. “The vast majority of cats with the condition have a noncancerous adenoma on their thyroid gland that causes it to secrete extra thyroid hormone. A tiny minority have a thyroid carcinoma, which is a cancerous tumor.”

According to Cohn, age is the only known risk factor. “I’ve never seen hyperthyroidism in a cat younger than 6 years old,” she says. “Typically, we start to see it around age 12 or 13.” 

Recognize the Signs of Hyperthyroidism in Cats

Thyroid hormone stimulates the metabolism, which is the process by which what a cat eats and drinks is converted into energy. So cats with hyperthyroidism (i.e. too much thyroid hormone) have hyped up metabolisms, Cohn says. “The major first sign that people will most often notice in hyperthyroid cats is that they lose weight despite having a good appetite,” she continues. “It’s also common for owners to say their cat is suddenly much more playful and active and is meowing a lot. These signs can be confusing because they can make the cat appear healthy.” 

But while losing weight and gaining energy may sound like a good thing, hyperthyroidism can lead to serious secondary problems. Most importantly, says Cohn, the cat’s hyped up metabolism forces the heart to work harder than it should. This can lead to a type of hyperthyroid heart disease where the heart muscle thickens and makes pumping blood more difficult, which can cause the heart to fail.

According to Cohn, other possible signs of hyperthyroidism that pet owners may notice include vomiting, diarrhea, and a decline in grooming habits leading to poor hair coat. The American Association of Feline Practitioners adds that cats with hyperthyroidism may be more thirsty and urinate more. 

Diagnosis and Treatment

Cohn says the physical exam can be an important step in diagnosing hyperthyroidism because in many cases, the veterinarian will be able to feel the tumor in the cat’s neck. The veterinarian may also be able to hear a heart murmur or gallop rhythm, due to secondary heart problems. “If I was examining an elderly cat and it exhibited these signs, I’d be pretty sure I knew what was going on,” Cohn explains. “But I wouldn’t know for certain until measuring thyroid hormone concentrations in the blood.” 

Once your cat has been diagnosed with hyperthyroidism, you'll need to discuss treatment options with your vet. There are basically four different treatment options for cats with hyperthyroidism:

Medication 

Cohn says cats can be given antithyroid medication in the form of a pill or a topical cream that’s applied inside the ear. “It doesn’t cure the condition,” she explains. “It just controls the amount of thyroid hormone produced.”

There are downsides, however. While medication therapy can be cost-effective in the short term, Cohn notes that it can be an expensive option for a middle-aged cat that has another ten years of life left. It can also be costly in terms of the owner’s time and energy, as the medication must be given twice a day for the rest of the cat’s life. And because these drugs can have negative side effects, Cohn says cats will need to have regular blood tests to make sure they aren’t being harmed.

Prescription Diet

Cats diagnosed early on in the disease process (through a routine blood test, for example) may be able to have their hyperthyroidism managed with nutritional therapy, Cohn says. This requires a prescription diet that has zero iodine, because iodine is used by the thyroid gland to produce thyroid hormone.

“This is only a viable option if the cat has early, mild disease,” she explains. “It’s not going to work for a cat that has heart disease.” Cohn also warns that diet therapy should only be considered if the owner is able to ensure the cat eats the prescription food and nothing else—no mice, no birds, no table scraps, no treats. All of these things will contain iodine and cancel out the special diet food. 

Surgery

Cats with hyperthyroidism can also undergo surgery to remove the thyroid tumor and glands, but there are definite downsides, Cohn cautions. For starters, performing surgery on an elderly animal that often has underlying heart disease is risky. And for the surgery to be successful, it’s crucial that the veterinarian removes all thyroid tissue, which can be difficult.

“Sometimes,” Cohn says, “there’s a little bit of thyroid tissue down in the chest, and if you don’t get that you’re not going to resolve the condition. And if you only take one thyroid gland (cats have two), the other gland can still cause hyperthyroidism.”

Moreover, Cohn notes when removing the thyroid glands, veterinarians can accidentally take out or damage the parathyroid glands as well, which are important for regulating calcium in the blood. With all of this in mind, she advises choosing another treatment option, if possible. “It’s still a viable possibility, but it’s not what most veterinarians would recommend,” Cohn explains. 

Radioactive Iodine Therapy

Radioactive iodine therapy, commonly called I-131, is the best treatment option in most scenarios, Cohn says. It involves giving the cat a small dose of radioactive iodine that concentrates in the thyroid glands and kills abnormal tissue without harming normal thyroid tissue. According to Cohn, 95 percent of cats are cured after one treatment. That means no more medications, no more treatments—the cat is back to normal. And in the 5 percent of cats who aren’t cured after one treatment, she says that the vast majority are cured after a second treatment.

Still, it’s not perfect. “There’s a possibility it won’t kill all of the abnormal thyroid tissue,” Cohn explains. “And there’s also a possibility the therapy will get too much tissue and the cat will end up with hypothyroidism. It’s rare, but it can happen.” 

Radioactive iodine therapy can only be administered in facilities that are specially licensed for the treatment. “It’s one of the reasons pet owners come to see us at the University of Missouri,” Cohn says. She explains that cats must stay in the hospital until they’re no longer considered a radioactive danger—a timeframe that’s entirely dependent on local laws and regulations. According to Cohn, it ranges anywhere from an overnight stay to a two-week stay. 

Cats with hyperthyroidism who are treated early can go on to live long, healthy lives. Cohn notes that her own personal cat was treated with I-131 at the age of 12 and lived another 10 years. 

Is it Possible to Prevent Hyperthyroidism in Cats?

Because age is the only known risk factor, Cohn says there isn’t much you can do to prevent hyperthyroidism in your cat. However, you can take steps to catch the condition before it does damage to your pet’s heart. “Once your cat gets a little older—around 8 or 9 years—regular physical exams with routine blood tests, including thyroid assays, can absolutely catch hyperthyroidism early,” she advises.