Symptoms & Treatment of Hyperthyroidism in Cats

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​Just like humans, cats have glands that control many bodily functions. One important gland, located at the base of the neck, is the thyroid. The thyroid​ regulates many functions, including metabolism.​ One of the first things you may notice about a cat with hyperthyroidism is that she is urinating a lot and may be eating a lot but doesn't seem to be gaining any weight.


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As the Merck Veterinary Manual explains​, hyperthyroidism means the thyroid produces too much hormone. The opposite disorder, hypothyroidism, means the body makes too little of the hormone​. Hypothyroidism causes sluggish metabolism while hyperthyroidism ​causes an unusually ​fast​ metabolism.​


Signs and symptoms​ of hyperthyroidism​

Hyperthyroidism is a common disease in cats​ that is usually caused by an enlarged thyroid gland​​. Symptoms usually appear around middle age or as the cat becomes a senior. A cat is considered middle aged, according to the Arlington Cat Clinic, at about 7 years of age.

Thyroid hormones affect nearly all of the organs in the body​, so there are a variety of symptoms of hyperthyroidism. At first, symptoms may appear to be subtle and you may not be sure that anything is wrong, but you may feel that your cat just doesn't seem right. One of the first and most obvious signs is a cat ​who is eating normally but not gaining weight, or even losing weight. He may appear malnourished, even though he's eating normally.


Your cat may be drinking more water, which in turn means more urination. Her fur may look like she's no longer grooming it—​an ​unkempt, matted or greasy ​appearance is common.​ According to Washington State University College of Veterinary Medicine​, your cat may be pooping more and it may be soft and more like diarrhea.

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​D​iet​ for a cat with hyperthyroidism

One aspect of overactive thyroid hormone production is that the thyroid produces ​two hormones, called T3 and T4​ ​​, which contain iodine. Prescription food is available for cats which contains little iodine.


According to Merck, dietary treatment works best for cats with only mild hyperthyroidism​. And it is important to remember that the cats must eat this prescription food, and only this prescription food, for the rest of their lives. If they eat other foods or treats that contain iodine, it will reduce the effectiveness of the treatment.​

T​reatment​ for cat hyperthyroidism

The most common way to treat a cat with hyperthyroidism is with a d​aily pill of methimazole, which blocks the production of thyroid hormone. Your veterinarian will do some blood tests to determine a good level treatment to start with, and then will probably ask you to give the pill for a few days and then will test your cat's thyroid levels again. It may take a little adjusting to be sure that the right amount of medicine is being given. For instance, you may give one whole pill in the morning, and a half a pill at night.


Many cats who are taking pills daily will develop a resistance to wanting to take the pills. In the case of hyperthyroidism, there is a medication which blocks thyroid production that is available in a gel that can be applied to the skin. This involves using a little plastic glove on the fingertip and scooping up a little of the gel and rubbing it inside the cat's ear. The Cornell University College of Veterinary Medicine says this is an acceptable effective treatment in most cases.​​

Surgical treatment for overactive thyroid

It is possible for a veterinarian to remove the thyroid gland through a surgical procedure. ​Cornell says this surgery has a good success rate​ although it is considered risky for older cats with ​issues that might be complicated by being under general anesthesia such as ​heart​ issues. ​Although surgery works, surgery is usually not the first ​treatment ​for this because the medication works so well.


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Radioactive iodine therapy

Another treatment option is to give radioactive iodine. The radioactive iodine destroys the abnormal, enlarged thyroid tissue so hormone levels can go back to normal. This is given as an injection, and Cornell says this results in normal hormone levels within one to two weeks of treatment. The problem with this is that since the treatment is actually radioactive, only veterinary facilities that are specially licensed to handle radioactive substances are allowed to use it.


The cat must also remain in the hospital until its radiation level is within acceptable levels to allow it to go home. Also, no visitors are allowed out of fears of exposure to radiation. Because of this expense and concern, many people opt for the daily medication, which is fairly affordable.

Hyperthyroid cat ​life expectancy​

As stated above, symptoms of hyperthyroidism in cats usually appears at about 7 years of age.​ It is difficult to estimate life expectancy because much of the outcome depends on how old the cat was when the hyperthyroidism was diagnosed, the seriousness of the overactive hormone and whether the condition was treated or not.


According to All Feline Hospital, hyperthyroidism cam cause heart problems, kidney disease and high blood pressure, among other symptoms, especially if it is left untreated. To get the most time with your cat that you can, begin and continue thyroid treatment. Once your cat's thyroid hormone is back in normal range, the symptoms you originally noticed should clear up and your cat may be healthy once again.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.