Hyperthyroidism of Cats

Hyperthyroidism is the most common hormone imbalance of cats, Hyperthyroidism, the overproduction of thyroid hormone, is relatively common in aging cats and may explain changes in behavior, weight loss, vomiting and diarrhea, among other signs, There are various clinical signs or “symptoms” that a cat with hyperthyroidism may display.  Some of the most common signs include weight loss, changes in appetite, gastrointestinal disturbances (e.g., vomiting and/or diarrhea), and changes in urination.  Affected cats may also show changes in activity level and attitude.  If you ever notice any of these signs in your cat, schedule an appointment with your veterinarian immediately.  These clinical signs are caused by the over-production of thyroid hormone by the cat’s thyroid glands, which are small endocrine glands located on the cat’s ventral neck, just adjacent to the windpipe (trachea).


In many cases, a veterinarian will become suspicious of hyperthyroidism based on a history of common clinical signs and findings from a complete physical examination.  Some of the most common findings on physical examination include evidence of weight loss, dehydration, and the presence of a heart murmur.  In some cases, the veterinarian will be able to feel a lump or enlargement on the thyroid gland itself.  To confirm a diagnosis of feline hyperthyroidism, blood tests are used to measure the levels of thyroid hormone circulating in the cat’s blood, which will be elevated in affected cats.  Remember that all cats need to have a veterinary examination performed at least once each year so that diseases like hyperthyroidism can be identified as early as possible.


There are a few different treatment options available for cats with hyperthyroidism, all of which have their advantages and disadvantages.  Traditionally, the most common method for controlling hyperthyroidism is the administration of anti-thyroid medications that help to suppress the production of excessive thyroid hormone.  These medications are often effective and are relatively inexpensive, however life-long treatment is required and some cats will develop significant side effects.  A more definitive treatment option is also available, which involves the administration of a radioactive iodine injection to the cat. The radioactive iodine is only absorbed by the overactive thyroid tissue, which results in the destruction of this abnormal tissue.  This procedure is very safe and typically very effective, however it is associated with higher initial costs and requires the cat to be hospitalized in a specialized facility for several days.  More recently, a new therapeutic diet was developed for cats with hyperthyroidism.  The therapeutic food has restricted levels of iodine, which is an essential component for thyroid hormone production.  Therefore, by limiting the cat’s intake of iodine you are also able to limit the amount of thyroid hormone that the cat can produce.  This option offers the ease of providing treatment by simply feeding the cat, however, the cat’s diet must be strictly limited to the prescription food, which can sometimes present a challenge in multi-cat households or in cats with picky appetites.  Finally, surgical removal of the thyroid gland itself is also a method for treating hyperthyroid cats.

No matter which treatment option is selected, many hyperthyroid cats can be effectively managed and will often show improvement, if not resolution, of their clinical disease.  Nevertheless, long-term monitoring and regular veterinary checks are important for affected cats.  If you have any questions about this disease or if you think your cat may be showing signs consistent with hyperthyroidism, contact your veterinarian immediately.

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