Hyperthyroidism is an increasingly common endocrine disorder in cats, particularly in those older than 8 years of age. The thyroid gland, which is located in the neck, can become enlarged and produce excessive hormones. This disorder may also cause enlarged nodules in the thyroid. These are usually benign, but may on rare occasions be cancerous (1-2% of cases). Cats with hyperthyroidism seem unable to rest and become hyperactive, pacing and grooming constantly. Despite the attention, the coat appears dull and unkempt. Most cats with the disease lose weight even though their appetite is normal, sometimes even voracious. Other signs include vomiting, diarrhea and excessively long claws. As the disease progresses, the cat becomes thinner and more run-down.
Some studies reported a twofold to threefold increase in risk of developing hyperthyroidism among cats fed mostly canned cat food (especially food consumed from pop top cans). Cats that preferred fish flavored or liver or giblets-flavored cat food had an increased risk of developing hyperthyroidism. There was also a threefold increase in risk among cats using cat litter. Exposure to fertilizers, herbicides, plant pesticides, or flea control products or the presence of a smoker in the home were not significantly associated with an increased risk of developing hyperthyroidism. Other identified risk factors were consumption of baby food and increased frequency of carpet cleaning.1
One of the problem with this disease is that most hyperthyroid cats have signs for longer than 6 months before the owners seek veterinary assistance. The primary reason for this delay is that initially cats maintain good (sometimes ravenous) appetites and are active and even overactive pets. These changes are often interpreted as evidence of health, not disease. More worrisome signs also begin slowly and are seen infrequently. The owner, therefore, typically sees a pet that "appears healthy" during the initial stage of the disease. It is not until weight loss and some other worrisome signs appear that an owner realizes that her cat is sick.2
Hair coat changes among hyperthyroid cats include patchy alopecia, matted hair and greasy seborrhea. Excess dander may also be seen. Decreased grooming and unkempt coat may lead to bacterial skin infections, although this is not common. Cats may experience outbreaks of demodicosis. Increased protein intake can lead to nail overgrowth.5
Hyperthyroidism is responsible for severe disturbances in other organs. It may cause congestive heart failure. Heart problems related to the disease disappear once hyperthyroidism is treated. Conversely, if the cat is left untreated, it will almost certainly die. Hyperthyroidism can mask preexisting kidney disease. In up to 40% of treated cats significant changes in kidney function only appear after treatment of hyperthyroidism.
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Hyperthyroidism is generally diagnosed by a blood test measuring the amount of thyroid hormones in the blood. One or both thyroid glands may be diseased, although in nearly all instances of hyperthyroidism, both glands are affected. There are several effective treatments, including oral medication, surgery to remove part of the thyroid gland, and a dose of radioactive iodine. Surgical treament requires prior medication to control the hyperthyroidism and make the cat a better surgical candidate. Thyroid surgery is generally successful.3 The veterinarian's recommendation will depend on the age and condition of the cat.
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