Hyperthyroidism in Cats: Causes and Diagnosis

Although hyperthyroidism is common in cats, its exact cause remains a mystery. Several factors are thought to contribute to its development, but often, no direct cause can be identified. If your cat is over 10 years old and showing any cat thyroid symptoms, your veterinarian may recommend blood tests to check thyroid hormone levels.

Causes of Hyperthyroidism in Cats

Hyperthyroidism is one of the most common diseases seen in older cats. But what exactly causes thyroid disease in cats? While there is no single answer, researchers believe the condition develops because of a combination of factors. Understanding these factors can help you be proactive about your cat’s health.

Environmental and Dietary Factors

Some environmental and dietary factors may play a role in cats developing hyperthyroidism. However, it’s important to note that these connections remain unproven. And because there have been no long-term studies confirming a direct cause, these are not facts and only considered possible associations. Chemicals like bisphenol A, phthalates and flame retardants have raised concern for their potential effects on thyroid function. Diet type and certain ingredients have also been suggested as possibly playing a role in hyperthyroidism in cats. Talk to your veterinarian about practical ways to support your cat’s thyroid health.¹

Genetic Factors

Genetics may play a role in a cat’s risk for developing hyperthyroidism. One small study found that Siamese and Burmese cats appear to have a lower risk.¹ However, larger studies are needed to confirm this finding.

Age-Related Changes

Hyperthyroidism is most common in senior cats, particularly those over 10 years old. As cats age, changes in the thyroid glands can increase the risk of hormone overproduction, making regular checkups especially important. Since the 1970s, more cats are living indoors, eating commercial diets and enjoying longer lives thanks to better veterinary care. While these changes have improved overall health, cats living longer lives and some potential lifestyle risk factors may also play a role in the rising number of hyperthyroidism cases.¹

How to Confirm Cat Thyroid Problems: Veterinary Diagnosis¹

If your veterinarian suspects hyperthyroidism, they will start with a full physical exam. This includes checking your cat’s weight, heart rate, breathing and the neck area for an enlarged thyroid gland. Your veterinarian will also listen for heart murmurs or irregular rhythms and assess coat quality, which may appear rough or poorly groomed. An eye exam may be performed to look for signs of high blood pressure that can affect the eyes.

Testing for Thyroid Disease in Cats

Your veterinarian can guide you through which tests are best for your cat and explain what the results mean for their care. To confirm a diagnosis of hyperthyroidism in your cat, your veterinarian may recommend blood and urine tests, including:

• Routine bloodwork and urinalysis – These tests help assess overall health and rule out other conditions like kidney disease. This information is essential for your veterinarian to help determine the best treatment or management strategy for your cat (which may include a cat overactive thyroid diet).  
• Thyroid hormone levels – Elevated levels confirm the diagnosis when paired with typical signs. Although hyperthyroidism is most often diagnosed in senior cats, it can sometimes be found in younger cats. It is important to understand that T₄ hormone levels tend to decrease as cats age. This means that a senior cat with a high-normal T₄ level may actually be in the early hyperthyroidism stages. In cats, tracking T₄ levels each year can help identify changes. If T₄ increases over time and your cat begins to show clinical signs, your veterinarian may recommend further testing, even if the result still falls within the normal range.²
• Advanced thyroid testing – If regular tests are unclear, additional tests such as a free T₄ test, a T₃ suppression test or thyroid imaging (scintigraphy) may be needed.
• Blood pressure measurement – High blood pressure is common with hyperthyroidism. Your veterinarian may also check your cat’s eyes for signs of damage from high blood pressure.
• X-rays or ultrasound – In some cases, chest X-rays, an ultrasound or an echocardiogram may be recommended to evaluate the heart or check for other health issues.

For hyperthyroidism in cats, treatment is recommended, even for cats with other conditions like kidney disease. Before starting, your veterinarian will check kidney function, blood pressure and urine protein to help determine the safest option. By working with your veterinarian, you can feel confident you’re choosing the tests and treatments that are best for your cat.

For more expert tips on hyperthyroidism in cats, explore the treatment options.

References

1. Carney HC, Ward CR, Bailey SJ, et al. 2016 AAFP guidelines for the management of feline hyperthyroidism. J Feline Med Surg. 2016;18(5):400-416.

2. Bugbee A, Rucinsky R, Cazabon S, et al. 2023 AAHA selected endocrinopathies of dogs and cats guidelines. JAAHA. 2023;59(3):113-135.