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Hyperthyroidism in Cats: Causes, Symptoms, and Treatment
Cat DiseasesCat Health

Hyperthyroidism in Cats: Causes, Symptoms, and Treatment

Hyperthyroidism is the most common endocrine disease in middle-aged and older cats. Its occurrence is far more rare in younger felines.  

There is no specific breed with a higher genetic predisposition for hyperthyroidism, but a few breeds, including Persian, Siamese, and Burmese kitties, seem less inclined to develop the condition.

Hyperthyroidism can greatly affect your cat's overall health and well-being, so it's important to understand what hyperthyroidism is and what you can do as a pet parent to recognize and treat it.

What Is Hyperthyroidism in Cats?

Your cat has two thyroid glands in the neck that are essential to control their body's metabolic rate. Hyperthyroidism occurs with the overproduction of thyroid hormone, which causes a rise in the animal's metabolism.

Although hyperthyroidism causes the thyroid gland to enlarge, there is a benign (non-malignant) cause in most cases. Fewer than 2% of cats with hyperthyroidism have malignant tumors. 

Several vital organs are negatively impacted by hyperthyroidism, including the heart, so pet parents need to be able to identify the warning signs of hyperthyroidism to keep their fur babies happy and healthy. 

a drawing of a cat sitting next to another cat
a drawing of a cat sitting next to another cat

Recognizing the Signs and Symptoms of Hyperthyroidism in Cats

According to PetMD, the most common symptom of hyperthyroidism in cats is weight loss. This results from a higher metabolism rate coupled with an increased appetite. Cats with hyperthyroidism can be hyperactive, irritable, or downright aggressive. They may increase their water intake, causing them to urinate more often and increase vocalization, especially during the night. Some cats suffer from occasional vomiting or diarrhea, and their fur may become matted and greasy. In more severe cases, anorexia can result as the disease progresses.

Causes of Hyperthyroidism in Cats 

Hyperthyroidism is a condition that affects mostly senior cats. According to VCAhospitals, the typical cat with hyperthyroidism is middle-aged or older; the average age of felines with hyperthyroidism is 12 years. Only about 5% of affected cats are less than ten years old.

Most of the time, an enlarged thyroid gland is caused by a noncancerous tumor, but in rare cases, hyperthyroidism can indicate the presence of malignant cancer. If your cat's neck glands feel swollen, consult your veterinarian immediately to rule it out.

With due diligence and a veterinarian's care, hyperthyroidism in cats can be treated successfully, and your pet will enjoy a long and healthy life.

Diagnosing Hyperthyroidism in Cats: What You Need to Know 

In most cases, diagnosing feline hyperthyroidism is pretty straightforward. There is a test your vet can order that measures the levels of thyroid hormone in your cat's blood, which can conclusively prove your cat is suffering from hyperthyroidism. 

Sometimes pinpointing the correct diagnosis can be more challenging, as other conditions can also cause thyroid levels to increase and mimic hyperthyroidism. Cats exhibiting signs of hyperthyroidism with thyroid levels in the mid-to-high-end range of normal may require additional blood work and imaging tests (these can include ultrasound or scintigraphy) before a definitive diagnosis can be made.

Treatment For Hyperthyroidism in Cats

There are several ways to treat hyperthyroidism in cats. The majority of cats are initially treated with an oral medication that contains methimazole. The medication can be given over the cat’s lifespan or used to stabilize the kitty prior to other treatment modalities, including radioactive iodine therapy or surgery.

Methimazole halts the production of the thyroid hormone but does not cure hyperthyroidism. It keeps the feline's symptoms under control but, unfortunately, does not cure the condition. Therefore, treatment will be necessary for the duration of your pet's life. 

After your cat has been stabilized with oral medication, your veterinarian will discuss further treatments that may also be an option for your pet. 

Dietary Changes for Hyperthyroidism in Cats 

Certain studies suggest that some cats with hyperthyroidism may benefit from limiting the amount of iodine in their diet. This treatment option can be utilized for cats with additional medical issues that render other avenues of care dangerous or even impossible. 

However, it is important to note that dietary restrictions are controversial due to concerns regarding the long-term effects of iodine restriction on a cat's overall health. 

Radioactive Iodine Therapy for Hyperthyroidism in Cats

During this treatment for hyperthyroidism, a radioactive iodine (I-131) injection is given to your cat, which is quickly absorbed into their bloodstream. According to Cornell University College of Veterinary Medicine, I-131 is absorbed by the thyroid gland, and the radiation destroys abnormal thyroid tissue without harming any healthy surrounding tissues or the nearby parathyroid glands. Most cats who are treated with radioactive iodine achieve normal thyroid hormone levels within one to two weeks of starting iodine therapy. 

The downside is that your cat must be hospitalized until the radiation level reaches safe limits. In most cases, this means your pet will require hospitalization for approximately three to five days after receiving this treatment. Because of strict safety guidelines, visitors are not usually allowed while the cat is in quarantine.

However, the advantages of radioactive iodine therapy are clear. The procedure usually eradicates hyperthyroidism without any serious side effects and does not require your cat to undergo anesthesia.

The treatment requires the handling and administration of a radioactive substance only allowed at practices specifically licensed to use radioisotopes. The radioactive procedure involves minimal risk for your pet, but protective action is required for the people who come in close contact with the animal until their radiation level drops. 

Surgery for Hyperthyroidism in Cats 

Surgical removal of the thyroid glands is a long-term or permanent cure in most cats, eliminating the prospect of lifelong medication. This makes surgery an option for many pet parents.

On the downside, this type of surgery requires general anesthesia. This always carries a certain risk, especially for senior cats with possible heart or kidney problems that could trigger complications like high blood pressure. 

One major risk of surgical thyroidectomy (removal of the thyroid) is causing inadvertent damage to the cat's parathyroid glands. This is due to the proximity of the parathyroid glands and the thyroid glands. Any harm to the parathyroid is to be avoided, as they are crucial for your cat to maintain adequate blood calcium levels. 

Medications such as Methimazole and radioactive iodine therapy (I-131) treat hyperthyroidism in cats as effectively as surgery and are far less invasive for your cat as well. 

Managing Hyperthyroidism in Cats: What You Can Do

All middle-aged and senior cats should receive a complete physical examination by a veterinarian every six months. Special attention should be given to the thyroid glands for evidence of thyroid enlargement and any other clinical signs of hyperthyroidism. Annual blood and urine tests are important for all cats seven years and older to detect problems before potentially irreversible damage occurs. 

Preventing Hyperthyroidism in Cats

There are currently no known preventive measures for hyperthyroidism, but early diagnosis decreases the incidence of secondary problems and improves the prognosis for your kitty. To avoid any problems, middle-aged and senior cats should receive a complete physical examination by a veterinarian every six months.

Pet parents should monitor their cat's behavior and make note of symptoms, including increased appetite and vocalization, sudden weight loss, and irritability. If you have an older cat, routinely check their thyroid glands for swelling as well. If your kitty exhibits any of these symptoms, call your vet for guidance.

Living with a Hyperthyroid Cat: What to Expect

If your cat's condition is diligently managed, the prognosis for feline hyperthyroidism is good, and most cats will survive many years and enjoy a good quality of life. If late-stage hyperthyroidism in cats is left untreated, the cat will suffer severe weight loss, heart disease, high blood pressure, and eventually death.


Monitor your pet for any signs and symptoms of hyperthyroidism and work closely with your vet. As a team, you can ensure your cat enjoys many more years of gleefully knocking glasses off coffee tables. 

Frequently Asked Questions

How can pet owners help their cats with hyperthyroidism? 

Pet parents can monitor their older cat's behavior and make note of symptoms such as increased appetite and vocalization, sudden weight loss, and irritability, and also routinely check their pet's thyroid glands for swelling. If your kitty exhibits any of these symptoms, call your vet for guidance.

What are the signs of hyperthyroidism in a cat?

Signs of hyperthyroidism in a cat can include weight loss (most common), increased appetite and vocalization, restlessness, and irritability. 

What happens to cats with untreated hyperthyroidism?

Cats with untreated hyperthyroidism suffer dramatic weight loss, progress to heart disease, and then death.