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HyperKitty : Overview of Feline Hyperthyroidism


Feline hyperthyroidism is the most common endocrine (hormonal) disorder of cats. It occurs most frequently in middle-aged and elderly cats. Cats fed fish-flavored canned cat food (particularly salmon) or liver and giblets-flavored canned cat food are at increased risk of developing this condition. Siamese and Himalayan cats are at decreased risk of hyperthyroidism compared to other breeds of cats.

Cats have two thyroid glands located in the neck. Feline hyperthyroidism is caused by the development of a growth on one or both thyroid glands. Most commonly, the growth is of a benign type called an adenoma; more rarely, a malignant tumor of the thyroid gland can occur. Affected glands produce an excess of thyroid hormones, which cause the heart to beat more rapidly and can result in heart damage and failure. Hypertension (high blood pressure) often develops. Hyperthyroidism can adversely affect the kidneys, and may cause elevated liver enzyme levels.

            Weight loss is the most common sign of feline hyperthyroidism. Increased appetite is frequently noted, and increased thirst may also be seen. Owners may feel the cat’s heart beating more rapidly. Affected cats may become restless or irritable. Diarrhea or vomiting can occur, but should raise suspicion that the cat may have gastrointestinal disease, such as inflammatory bowel disease or intestinal lymphoma.

            Veterinarians diagnose feline hyperthyroidism through blood tests and palpation of an enlarged thyroid gland. The most reliable test is measurement of a thyroid hormone called the total T4. Cats who have additional illness may have decreased levels of thyroid hormones, and this can make the diagnosis of hyperthyroidism more difficult. In this situation, it is most important to diagnose the disease which is depressing the thyroid hormone level. In cases where feline hyperthyroidism is suspected but total T4 levels are normal, a test called a free T4 level is often performed, but this test is unreliable since falsely elevated values may occur with non-thyroidal illnesses. Intestinal diseases may cause similar symptoms to hyperthyroidism, and should be suspected in cats who are losing weight and do not have elevated total T4 thyroid hormone levels.

            If a cat has a normal or low total T4 level, even if the free T4 level is elevated, it is essential to rule out intestinal disease as the cause of the cat’s symptoms. A good initial step is an abdominal ultrasound, which may show thickened intestines or enlarged intestinal lymph nodes. However, ultrasound findings can be perfectly normal in cats with even advanced intestinal disease. If a cat is losing weight and full blood work and ultrasound do not reveal a definitive cause, intestinal biopsies, such as via endoscopy, should be considered. This is particularly true if the cat does not have other signs of hyperthyroidism, such as an elevated heart rate and/or a palpably enlarged thyroid gland.

            Feline hyperthyroidism can be treated with medication, radioactive iodine therapy, or surgery. The most commonly used medication is called methimazole (Tapazole®). Use of this drug requires close monitoring of the cat’s kidney function, blood cell counts, and liver enzymes, because serious side effects can occur. Medication does not cure the disease but will decrease the hormone levels. Because the thyroid growth will continue to enlarge, the dose of medication often needs to be periodically increased. Research has shown that methimazole is more effective with fewer side effects if the dose is divided into twice-daily dosing. For example, for a cat receiving 5 mg a day, it is safer and more effective to give 2.5 mg twice a day.

            Radioactive iodine therapy (RAI) involves an injection of a radioactive isotope. Iodine is used because it is taken up by the thyroid gland. Research has shown that hyperthyroid cats receiving this treatment after initial regulation with medication have the longest lifespans. Since treatment of hyperthyroidism may affect kidney function, it is recommended that cats are treated with medication before receiving RAI, to ensure that kidney function will remain adequate when thyroid hormone levels normalize. It is also essential to closely monitor kidney values after RAI treatment.

Laws vary a bit from state to state, but it is required that cats receiving RAI are boarded in special facilities after the injection until it is deemed safe for them to be in close contact with their owners. Generally, cats thrive in these facilities, since most are designed for maximum feline comfort.

            Surgery to remove one or both thyroid glands may be performed. As with RAI, cats should first be treated with medication if possible to ensure that kidney function will remain adequate. Additionally, normalization of thyroid hormone levels before surgery with medication will decrease the risk of anesthetic complications, since elevated thyroid hormone levels can increase the risk of heart arrhythmias. Because the parathyroid glands, which control blood calcium levels, are located in close proximity to the thyroid glands, cats who have both thyroid glands removed may experience a dangerous drop in blood calcium levels. For this reason it is not generally recommended to remove both thyroid glands simultaneously; rather only the gland which is visibly enlarged is removed.

            It is essential to measure blood pressure both before and after treatment of hyperthyroidism. Hyperthyroidism can cause hypertension; additionally, research has shown that many cats with initially normal blood pressure will develop hypertension in the months following treatment.

            In cats who have severe reactions to methimazole, but who cannot be treated with RAI or surgery, perhaps due to the owner’s financial restrictions, a drug called atenolol can be used to control heart rate and blood pressure, and to improve the cat’s mood. This drug will not slow weight loss, however.

            Cats with feline hyperthyroidism who are promptly diagnosed and treated have a good prognosis. Prognosis is best in cats who receive treatment with medication followed by radioactive iodine therapy.        


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My cat is already a soft ball full of problems. And this... Can't even imagine what would I do in that case...

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