In the past, the three most common treatment options for hyperthyroid cats were medical management, surgery, or radioactive
iodine (I-131) therapy. Now there is another option—dietary management.
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The thyroid gland is a small, paired gland located in the neck adjacent to the trachea. The gland produces hormones that are
critical to the proper functioning of many organs, including the heart, liver, brain, and kidneys. One of these hormones is
thyroxine (T4), a hormone involved with the body's metabolic rate. T4 is converted in the liver, producing triiodothyronine (T3), an active form of thyroid hormone that affects the metabolic rate of every cell in the body.
When these hormones are secreted in excess from the thyroid glands, which become enlarged, an animal develops a condition
known as hyperthyroidism. In most cases in cats, the enlarged thyroid glands are caused by a benign tumor called an adenoma.
Rarely, the cause is a type of malignant tumor called a thyroid adenocarcinoma.
Since thyroid hormones affect so many organs in the body, secondary disease processes can often be identified in addition
to hyperthyroidism. Excess thyroid hormones result in a faster heart rate and stronger heart muscle contractions. If left
untreated, this can compromise the function of the heart and eventually lead to heart failure.
Hypertension is another possible complication of hyperthyroidism. Untreated, additional damage may occur to other organs,
including the kidneys, brain, and eyes. Hypertension increases renal blood flow and the glomerular filtration rate, which
could mask concurrent chronic renal failure. Initially, the hypertension may need to be treated with drugs. But as the hyperthyroidism
is managed, the associated hypertension will often resolve, requiring no further treatment.
Hyperthyroid cats may also develop changes in glucose and insulin metabolism. Sometimes an untreated hyperthyroid cat can
develop diabetes mellitus. This may worsen over time, even after successful treatment of hyperthyroidism.
Prevalence and predilections
Since the disease was first described in the late 1970s, hyperthyroidism has become the most commonly diagnosed endocrine
disorder in cats. Typically, the cause is adenomatous hyperplasia. It's unclear whether the incidence of the disease is on
the rise or whether greater veterinary awareness, improved diagnostics, and an aging cat population account for the increased
Hyperthyroidism is most often diagnosed in cats older than 8 years of age, although it can be seen in younger animals. No
sex or breed predilection has been identified, nor have any definitive risk factors. Some proposed, yet unproven, risk factors
include feeding canned food from pop-top cans, ectoparasiticide exposure, and the high iodine content of some cat foods. Another
theory is that immunologic, infectious, nutritional, environmental, or genetic factors may interact, causing pathologic changes.
Despite many theories, the pathogenesis of hyperthyroidism remains unknown.