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July 22, 2021 – Hyperthyroidism is one of the most common diseases affecting older cats with some estimates suggesting 10% of cats 10 years and older affected. Because of its prevalence, many cat owners know something about hyperthyroidism even if they haven’t had a cat diagnosed with it - yet.

The science around hyperthyroidism is an active area of research with constantly changing information. It’s important for cat owners to keep up on the latest regarding diagnosis and treatment to help keep their cats healthy and feeling good.

Thyroid function basics

To understand the whys behind the diagnostic tests used to detect abnormal thyroid function, as well as treatment, we’ve got to go back to the basics of thyroid function.

There are two thyroid glands, one located on each side of the throat, just below the voice box. Normal glands are so small you can’t detect them on palpation, but in some cats with hyperthyroidism the glands enlarge and sometimes can be detected during physical examination.

The thyroid plays an important role in regulating many different metabolic functions throughout the body. Several hormones are involved in the complex regulation of thyroid function, but the most important ones for owners to know about are T4 (also known as thyroxine), T3 and thyroid-stimulating hormone (TSH).

When the pituitary gland, located at the base of the brain, senses low blood thyroid hormone levels, it releases TSH. TSH travels to the thyroid gland where it stimulates production of T4 and T3. Once adequate blood levels of thyroid hormone are reached, the pituitary stops producing TSH. The process starts again when T4 and T3 levels decrease. This feedback loop is important to understanding both the diagnosis and treatment of hyperthyroidism

What goes wrong?

For reasons still unclear, thyroid cells begin to multiply, sometimes forming a benign growth, in either one or both thyroid glands. These tumors produce T3 and T4 independently of blood thyroid hormone levels. The thyroid no longer responds to normal feedback directing it to stop producing hormone. This constant supply of thyroid hormone speeds up the metabolic rate, which in turn results in some of the most common signs we see in cats with the disease.

Many theories exist about why cats develop hyperthyroidism. Genetics, diet and environmental exposures all have been proposed as potential causes of disease. This is an active area of research that could one day provide information to help cat owners prevent disease.


Determining whether a cat has hyperthyroidism rests on a combination of clinical signs and special testing.

The most common clinical signs reported in cats reflect the high metabolic rate secondary to increased thyroid hormones in the blood. They include:

  • Increased appetite
  • Weight loss (in spite of increased appetite)
  • Increased water consumption (with increased urination)
  • Vomiting
  • Diarrhea
  • Hyperexcitability
  • Vocalization

Untreated hyperthyroidism can take a major toll on many internal organs, especially the heart. Occasionally, cats are first diagnosed with hyperthyroidism because they experience heart failure – a bad combination that’s difficult to treat.

Several tests are used to make a diagnosis of hyperthyroidism. The most common include:

  • Total T4 measurement – Many routine bloodwork panels now include this test to screen cats for hyperthyroidism. It remains one of the best tests for detecting the disease. This test looks at the total amount of T4 in the blood.
  • Free T4 by equilibrium dialysis – This is a different way of measuring T4 and can be used in cases where the diagnosis is challenging.
  • TSH measurement – In cats with hyperthyroidism, TSH levels are very low. This helpful test also can be used to guide treatment.
  • Radionuclide thyroid scan – This test can be used to see whether one or both thyroid glands are enlarged and can be used in hard-to-diagnose cases.

Because measuring total T4 is becoming commonplace, many more cats are diagnosed with hyperthyroidism earlier in the course of their disease. While early diagnosis and treatment can be a good thing for your cat, it can make diagnosis a bit less straightforward. Your veterinarian can help decide which test(s) are best for your cat (and your pocketbook).


Four treatments are available for cats with hyperthyroidism - medication, radioactive iodine therapy, surgery and diet modification – and are used depending on a number of factors.

Medication – Methimazole is the drug of choice for treating cats with hyperthyroidism. Methimazole blocks the production of thyroid hormones but needs to be given every day to be effective. In the short term, methimazole can be the most cost-effective option for owners. Giving medication to cats can be challenging, but methimazole comes in a topical form that is easier to use. The drug has side effects that can limit its use in some cats, but for many cat owners it is a safe and effective therapy.

Radioactive iodine therapy – This treatment targets the tumor tissue specifically. Treatment results in destruction of the tumor. Some studies show that this therapy has the highest cure rate of all treatment options. Costs can be high with this therapy.

Surgery (thyroidectomy) – Surgery also can result in a cure if all the diseased tissue is removed. Radioactive imaging might be needed to help guide the surgeon toward the involved tissue (especially if it isn’t clear which gland is involved). In rare cases, abnormal thyroid tissue can be found within the chest cavity – if this tissue isn’t removed, the condition can persist. Removal of tissue from the chest cavity can be very difficult and requires an experienced surgeon. This treatment is costly but can be curative. Because of the effectiveness of radioactive iodine therapy, thyroidectomy is rarely recommended or performed anymore.

Diet – Hill’s Pet Nutrition y/d diet is an iodine-restricted formulation used to treat hyperthyroidism over the long term. Like medication, the diet must be fed exclusively every day, and the diet cannot be fed to normal cats that live in the same household.

It’s very important to work with your veterinarian to select which option works best for your cat. Treatment is not a one-size-fits-all proposition, especially if you have a kitty that has concurrent medical problems, such as chronic kidney disease.

Lastly, all cats require some long-term monitoring, even if they’ve had surgery or radioactive iodine therapy. It was thought for many years that hypothyroidism (low thyroid function) was rare after treatment due to small remnants of normal thyroid tissue remaining after therapy. We now know this assumption is incorrect, and some cats might need some hormone replacement therapy for the rest of their lives. Long-term follow-up also is critical for cats suffering from concurrent illnesses.

Do dogs get hyperthyroidism?

Dog owners are more familiar with hypothyroidism. Although rare, occasionally dogs can develop hyperthyroidism. Unfortunately, unlike their feline counterparts, hyperthyroidism in dogs is almost always associated with malignant tumors of the thyroid.

Rarely, over-supplementation with thyroid medication in hypothyroid dogs can lead to signs consistent with hyperthyroidism and it’s been reported that dogs eating thyroid tissue can develop hyperthyroidism.

For more information, you can take a deeper dive into the diagnosis and treatment of hyperthyroidism by listening to our Fresh Scoop podcast interview with veterinary endocrinologist Dr. Ellen Behrend.

Hyperthyroidism is a common, but treatable, condition of cats. There are many options available to owners when it comes to therapy and the prognosis for cats with the condition can be excellent.