Understanding Hyperthyroidism in Cats

Hyperthyroidism is a common disease in cats. Fortunately, there are signs to watch for and many treatment options available.

Jun 2, 2023By Heather Jarek, BA Veterinary Technology
hyperthyroidism in cats

Feline hyperthyroidism is the most common endocrine disorder in cats worldwide. It is most common in older cats over the age of 7. It’s estimated that 10% of cats will develop this disorder as they age. This disease is so common that veterinarians recommend routine screening as cats enter their golden years. It’s still important to be aware of the signs and symptoms of the disease as any cat, young or old, can develop it. The faster feline hyperthyroidism is diagnosed, the sooner your cat can return to living a happy, healthy life.

What is Feline Hyperthyroidism?

gray senior cat laying down

Hyperthyroidism in cats is a disease that affects a cat’s thyroid glands. These glands are in the neck and found on either side of a cat’s trachea. The thyroid glands are essential in regulating a cat’s metabolic rate. When a cat is hyperthyroid, its thyroid glands produce too much thyroid hormone, causing its metabolic rate to increase.

Signs of Hyperthyroidism in Cats

cat drinking water faucet

Cats suffering from hyperthyroidism can exhibit a variety of symptoms. Early disease symptoms are also the most common, including weight loss, increased appetite, increased thirst, and urination. As the disease progresses, the symptoms will become more severe, including vomiting, diarrhea, and an unkempt hair coat. Owners may notice their cat becoming hyperactive, restless, or even more aggressive.

Secondary complications may arise if the disease remains untreated. These include hypertension (high blood pressure) and thyrotoxic cardiomyopathy, a form of heart disease. These two secondary conditions can be reversed once the hyperthyroidism is successfully treated.

What Causes Feline Hyperthyroidism?

cat eating food from bowl

An increase in the production of thyroid hormones causes hyperthyroidism in cats. This usually occurs when the thyroid gland has become enlarged by the presence of a tumor. In most cases, these are non-cancerous growths called adenomas. In rare instances, these tumors can be cancerous and are known as thyroid adenocarcinomas.

The cause of these tumors and hyperthyroidism in cats is unknown. Chronic exposure to certain environmental or food chemicals may put a cat’s healthy thyroid glands at risk. Another potential cause could be an excess or deficiency of certain compounds in a cat’s diet. Experts believe dietary iodine may contribute to the disease, but research is still needed to confirm.

How is it Diagnosed?

feline blood draw

It’s vital to schedule a visit with your veterinarian if you suspect your cat may be suffering from hyperthyroidism. After a complete physical exam, your veterinarian may recommend testing your cat for the disease. The test consists of a blood draw to determine the level of thyroid hormones present in your cat’s bloodstream. In most cases, a cat with hyperthyroidism will have high levels of the thyroid hormone total thyroxine (TT4). Sometimes these levels are borderline, and a second thyroid test may be needed to confirm. It’s important to note that your cat may need several blood draws, or a short hospital stay to complete these tests.

If the tests come back as undiagnostic, but hyperthyroidism is heavily suspected, your veterinarian may recommend a thyroid scan. This scan will determine if there are any tumors affecting the thyroid glands. Your veterinarian may also recommend waiting a few weeks before rechecking the thyroid hormone levels.

What are the Treatment Options?

cat getting pills medication

Once a diagnosis is made, more testing may be in order and may include more blood tests, chest x-rays, a urinalysis, an ECG, and a blood pressure evaluation. These tests will help determine your cat’s overall health and check for any secondary complications caused by the disease. Your veterinarian may refer your cat to a specialist for an ultrasound of the heart if there are any heart disease concerns.

Treatment options will vary depending on each cat but consist of the following:

  • Surgery
  • Oral medication
  • Radioactive iodine
  • Prescription diet
  • Continued testing and monitoring

What is the Prognosis?

happy cat orange tabby

Less than 2% of feline hyperthyroidism cases are cancerous. This makes the overall prognosis for the disease excellent. Once the recommended treatment is complete, most cats go on to lead healthy lives. Some cats may need permanent medications or prescription diets to maintain healthy thyroid levels. Routine testing and blood work may be required to check thyroid levels or monitor any secondary disease complications.

Which Cats are at Risk of Developing Hyperthyroidism?

siamese cat laying window

Any age or breed of cat can develop hyperthyroidism. Middle-aged to older cats are most at risk for developing the disease. Some cats may be at a higher risk due to environmental or diet factors, but research on this theory is still being conducted.

No determined breeds are at a higher risk of developing feline hyperthyroidism. Siamese, Burmese, Persian, Himalayan, and Abyssinian breeds were found to have a lower incidence of developing the disease.

Can Hyperthyroidism in Cats Be Prevented?

cat veterinary examination

There is no recommended protocol for preventing hyperthyroidism in cats. Fortunately, there are ways to improve the prognosis for a hyperthyroid cat. Early diagnosis is the most critical factor in decreasing the chances of severe illness and secondary infections. Knowing your cat’s normal behaviors will help you recognize the earlier symptoms of the disease.

Routine veterinary visits are essential. Most vets recommend routine bloodwork once or twice a year, depending on your cat’s age. Older cats should have their thyroid levels checked yearly for early detection.

Heather Jarek
By Heather JarekBA Veterinary Technology

Heather is a lover of all animals, big and small. She graduated with a bachelor’s degree in veterinary technology in 2014. She has been working as a licensed veterinary technician for the last eight years. Her favorite hobby is horseback riding, and she has been riding horses since the age of eight. She enjoys spending time with her family at the lake with their golden retriever Calista in her free time.