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Hyperthyroidism (also called thyrotoxicosis) is one of the most common diseases of middle-aged and older cats. It is a multisystem disorder that develops when the thyroid gland becomes enlarged due to growth of a benign tumor (called a thyroid adenoma) and increases the amount of thyroid hormones (T3 and T4 hormones) produced. It was first documented in cats almost 30 years ago, but the cause of thyroid adenomas has been elusive.


  • Weight loss
  • Increased appetite
  • Vomiting
  • Increased thirst and urination
  • Hyperactivity
  • Diarrhea
  • Difficulty breathing

Risk Factors

Hyperthyroidism is one of the most common diseases of middle-aged (about seven years old) and senior cats (older than 10 years). Environmental and dietary risk factors have been investigated and may play a role in predisposing cats to hyperthyroidism, though the specific factors and mechanisms are not known. No individual breed is known to be at increased risk.


Your veterinarian will conduct a physical examination that includes palpating the neck area for enlarged glands. Your cat’s heart rate and blood pressure may also be checked. If hyperthyroidism is suspected, your veterinarian will likely order a blood-chemistry panel and will test the thyroid-hormone levels. Most cats with hyperthyroidism will have elevated levels of the thyroid hormone T4 in their bloodstream. A small percentage of cats with hyperthyroidism, however, will have T4 levels within the normal range. If that is the case, but hyperthyroidism is still strongly suspected, your veterinarian may recommend additional tests to help confirm the diagnosis.

Because hyperthyroidism can predispose a cat to other conditions, it is important to evaluate general health, paying particular attention to the heart and kidneys. The blood-chemistry panel and urinalysis will reveal information about the function of other organs and provide your veterinarian with an overall picture of your cat’s health.


There are three treatment options for feline hyperthyroidism: medication, surgery and radioactive-iodine therapy. Each has advantages and disadvantages. Generally, the treatment a cat receives for hyperthyroidism will depend on specific circumstances, including heart and kidney function and age. Concern about kidney failure is a major determinant of the course of treatment and may eliminate some of the options.


Antithyroid drugs reduce the production and release of thyroid hormone from the thyroid gland. These medications do not cure the disease, but they do allow short-term or long-term control of hyperthyroidism. The advantages of medication are that the drugs are readily available and relatively inexpensive. Some cats may experience side effects, however, that include vomiting, anorexia, fever, anemia and lethargy. Lifelong treatment usually involves twice-daily oral dosage, and for some owners and cats, that dosage schedule may be difficult to achieve. Routine blood tests should be done periodically during treatment to evaluate the effectiveness of therapy, to monitor kidney function and to look for side effects.

Surgical Treatment

Removal of the thyroid glands, called surgical thyroidectomy, is a surgical procedure that has a good success rate if all the abnormal thyroid tissue can be found. Unfortunately, sometimes this tissue is located in difficult-to-reach places such as inside the chest cavity.  The advantage of surgery is that it is likely to produce a long-term or permanent cure in most cats, and therefore eliminates the need for long-term medication. This surgery requires general anesthesia, and there might be added risks if older cats have heart, kidney or other problems that could cause complications.

Radioactive-Iodine Therapy

Where it is available, radioactive-iodine therapy is quickly becoming the treatment of choice for cats with hyperthyroidism. During treatment, radioactive iodine is injected and quickly absorbed into the bloodstream. The iodine is taken up by the thyroid gland but not by other body tissues. The quantity of radiation destroys the abnormal thyroid tissue but does not damage the surrounding tissues or the parathyroid glands. Most cats have normal hormone levels within one to two weeks of treatment. The advantages of radioactive-iodine therapy are that the procedure is curative, has no serious side effects and does not require anesthesia. The downside is that cats must remain in quarantine at the veterinary facility until they reach a safe and legal radiation level—usually within three to five days. Some litter box precautions are required for about two weeks after treatment. 


The outcome of both medical management and radiation therapy is usually very positive, and most cats with hyperthyroidism have a very good chance of returning to an excellent state of health for many years.

Current Research

Click here to find out about Morris Animal Foundation–funded research into feline diabetes.

Morris Animal Foundation (MAF) urges pet owners not to implement any suggestions on animal health treatments without prior consultation with a licensed veterinarian. If your pet is experiencing health issues, contact your veterinarian. MAF does not endorse any of the medical treatments described in these videos. The Foundation funds research to enhance medical options available to veterinary professionals and their patients.