Background

Hyperthyroidism is a condition in which the body makes too much (hyper means “too much “) of the thyroid hormone thyroxine (also called T4). Feline hyperthyroidism is one of the two most common hormone imbalances in cats and affects about 10% of older cats. It is rare to see hyperthyroidism in cats less than eight years old. The average age at diagnosis is about 13 years. Multiple treatment options are available, and the long-term outlook is considered good to excellent.

The thyroid gland normally sits in the neck, usually below the voice box or larynx. Overall, the thyroid helps to regulate metabolic rate. In 98% of cats, the cause of hyperthyroidism is a benign thyroid tumor. In 2% of cats, however, the tumor is malignant at the time of hyperthyroidism diagnosis. A benign thyroid tumor can become cancerous over time.

Thyroid hormones affect many body systems, so there are a variety of symptoms. The most common sign is weight loss despite your cat eating the same amount and diet, or even eating more than normal. Cats with hyperthyroidism can have very good appetites, eating food they normally would not or stealing food. Vomiting can happen and, less commonly, diarrhea. A hyperthyroid cat may drink and urinate more than normal. Your cat may be more active, and their behavior can change.

On physical exam, your veterinarian can often feel the thyroid tumor. A tumor begins quite small, but sometimes you can feel it, too.

The cause of hyperthyroidism is unknown. Various factors have been studied -- genetics, diet, the presence of flame retardants in the home, etc. No single cause has been found, and none is likely to exist. In other words, many issues are at play.

Diagnosis

There are many causes for symptoms such as weight loss and vomiting, so the first step in determining a diagnosis will be to perform a complete blood count, a biochemical panel, and a urinalysis to identify possible causes. Lab work will also be needed to help determine the best treatment if your cat is hyperthyroid. 

To diagnose hyperthyroidism, specific thyroid function testing must be done. The first test is to measure the main thyroid hormone, thyroxine (T4). If T4 is above normal, symptoms of hyperthyroidism are present, and a thyroid tumor is felt, a diagnosis can be made. Often, measurement of T4 can make the diagnosis of hyperthyroidism without further thyroid testing. However, it is possible for T4 to be in a zone where the diagnosis cannot be made, but hyperthyroidism also cannot be ruled out. In such cases, measurement of a form of T4 called free T4 and/or measurement of thyroid-stimulating hormone (TSH) will be recommended. For measuring free T4, a method called the “equilibrium dialysis” assay is preferred.

Rarely, a diagnosis still cannot be made with a combination of T4, free T4, and TSH. Repeat testing or other tests will be recommended.

Treatment

There are four options for treating hyperthyroidism. 

• radioactive iodine
• surgical removal of part or all of the thyroid 
• medication
• dietary

All the treatment options have advantages and disadvantages. Which one is best depends on you and your cat. Radioactive iodine and surgery can potentially cure hyperthyroidism. Medical therapy with a drug called methimazole is often effective. Dietary therapy involves feeding a specific reduced iodine diet; the most commonly prescribed is Hill’s Prescription Diet y/d. Although medical and dietary therapy can often control the symptoms, neither option is a cure for hyperthyroidism, and both must be given lifelong. Medical therapy is more effective than dietary. With medication and dietary therapy, the thyroid tumor will continue to grow, and, after a few years or so of treatment, it can become much larger. It may also become cancerous.

Hyperthyroidism can mask kidney disease or kidney failure. In other words, the kidneys are not functioning normally, but hyperthyroidism helps them function better.

Treatment of hyperthyroidism with any method can help your veterinarian detect kidney disease. This occurs in about 20% of cats treated for hyperthyroidism, and the kidney disease identified is usually mild. Although some treatments for the kidney disease may be recommended, in the long term, allowing hyperthyroidism to support the kidneys will be bad for your cat’s health.

It is much better to treat the hyperthyroidism and mild kidney disease together.

If kidney disease is already diagnosed when the hyperthyroidism is found, treatment of the hyperthyroidism is more difficult. Trial medical therapy is recommended first to determine the effect of correcting the hyperthyroidism on the kidney function. Radioactive iodine and surgery may not be options.

Prognosis 

Overall, the outlook for cats with hyperthyroidism is excellent if the tumor is benign and the hyperthyroidism is controlled. Treated cats can live their normal lifespan and usually die of another disease. Survival is usually measured in years after diagnosis. With medical and dietary therapies, treatment is lifelong.

If the tumor is malignant, the outlook is not as good. Medical and dietary therapies are often not effective. 

If the hyperthyroidism is not treated, survival can be short. The number of symptoms increases, and they worsen. Heart failure and other severe complications can develop.