Hyperthyroidism occurs when there is overproduction and excessive secretion of thyroid hormone by the thyroid gland. It is the most common disorder of the endocrine glands of cats. In most instances, hyperthyroidism results from a benign increase in the number of active cells in the affected gland (adenomatous hyperplasia, thyroid adenoma), but rarely it is caused by thyroid cancer (thyroid adenocarcinoma).
Hyperthyroidism has developed in cats as young as three years of age, but most cases occur in cats older than nine years.
SIGNS OF HYPERTHYROIDISM
Cats affected by hyperthyroidism may have many different clinical signs at once, since thyroid hormone affects the function of so many different organ systems in the body. Sometimes, though, only one or just a few symptoms predominate.
Weight loss despite a normal or increased appetite and hyperexcit- ability are seen most often. Many cats seem more restless and irritable than usual. About one out of five cats will have periods of decreased as well as periods of increased appetite. A few may even refuse to eat, but this is usually only in the advanced stages of the disease when the cat has become depressed and weak. Many hyperthyroid cats vomit and/or have diarrhea, drink a lot, and urinate excessively.
High levels of thyroid hormone increase the blood pressure, thereby indirectly increasing the heart’s work load. They also have a more direct effect on the heart muscle itself, increasing its demand for oxygen, and they cause an increase in the body’s metabolic demands on the heart.
Extremely rapid heart rates (greater than 240 beats per minute) and/or irregular heartbeats may result, further compromising heart function.
Affected cats may develop heart disease, or those with preexisting heart disease may develop heart failure if the hyperthyroidism remains untreated. High blood pressure associated with thyroid disease may aggravate existing kidney malfunction or initiate it. Some cats with thyroid overproduction develop excessive shedding that may be accompanied by fur matting or by visible thinning of the hair. Others seem to have abnormally rapid toenail growth.
BLOOD TESTS ARE NECESSARY TO DIAGNOSE THYROID DISEASE
The cause of feline hyperthyroidism is unknown, so no measures can be taken to prevent it. It is most important to be aware of the disease and to make an early diagnosis to prevent severe body changes that can be caused by excess levels of thyroid hormone.
If you think your cat is exhibiting signs of hyperthyroidism you can feel along both sides of his or her neck near the windpipe (trachea) to see if you can find an enlarged thyroid gland. It could be located anywhere from the level of the larynx down to the chest level (or even inside the chest).
Although many cats with hyperthyroidism do not have anything you can feel, any unusual lump under the skin in this area should prompt you to arrange for a veterinarian’s evaluation.
In addition to physical changes you and your veterinarian may detect on physical examination, blood tests that measure thyroid hormone levels are needed to diagnose hyperthyroidism. Urinalysis, complete blood counts, biochemical evaluation of the blood, and chest radiographs (X-ray pictures) are also usually needed to determine any adverse effects the abnormal hormone levels have had on other organ systems and to evaluate whether your cat (especially if aged) can undergo treatment. In some cases, special studies such as echocardiograms, electrocardiograms, and radionuclide thyroid scans that can locate the exact site and size of an enlarged thyroid gland are needed.
SEVERAL TREATMENTS ARE AVAILABLE FOR HYPERTHYROIDISM
Treatment for feline hyperthyroidism consists of antithyroid drugs, surgical removal of the affected tissue, or the administration of radioactive iodine-131 which specifically destroys the abnormal tissue. Many veterinarians use antithyroid medications only as temporary measures to stabilize a patient until a more permanent treatment is given, since these drugs can be associated with many serious side effects. Although it is very safe for the patient, radioactive iodine-131 can only be administered at special centers that are licensed to properly dispose of radioactive wastes. Therefore, access to this treatment is limited in some geographic areas. Your veterinarian can give you the name of a veterinary specialist who can administer radioisotope treatment should you need it for your cat.
Thyroid surgery can usually be performed in any well-equipped veterinary clinic, but it may be contraindicated in certain patients such as those with advanced heart and/or kidney disease. The best choice of treatment for your cat will need to be discussed with your veterinarian and based on several factors including treatment availability and cost, as well as the presence of physical problems in addition to thyroid malfunction. Cats treated adequately and early enough can regain perfectly normal thyroid function and normal general health even if they are elderly.