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Encyclopedia of Feline Veterinary Medical Information
Hyperthyroidism
Hyperthyroidism is the most common hormonal disease of cats. The overproduction of thyroid hormone can be the result of hyperplasia (increased activity of the gland for unknown reasons) or cancer. A small percentage of the cancers are malignant. This disease was not commonly recognized prior to the late 1970s. The reason for the increase in prevalence is not known.
Hyperthyroidism affects older cats most commonly. It is seen occasionally in cats as young as 4 years of age. The clinical signs include weight loss, increased activity, increased appetite, vomiting or diarrhea, increased vocalization, increased drinking and increased urination. In some cats, the disorder produces atypical signs such as depression, inappetance or weakness.
The increase in thyroid hormone causes the cat's heart to beat faster, often > 240 beats per minute. Heart murmurs may be present. Heart failure will occur in up to 10% of cats and heart damage occurs in most, although it is usually reversible with treatment of the hyperthyroidism. The haircoat may look scruffy. Enlarged thyroid glands may be found. Effects on the kidneys from the circulatory changes can make existing kidney disease worse or cause the appearance of kidney disease in some cats.
Diabetes, kidney disease, liver disease and other conditions that also affect older cats need to be ruled out prior to settling on a diagnosis of hyperthyroidism, even if tests indicate it is present.
Testing for hyperthyroidism is done by measuring the T4 (one thyroid hormone) levels in the blood stream. In most cats with hyperthyroidism, these levels will be above normal. In some cats, they will be in the "normal" range, despite the presence of the disease. This is particularly true of very old cats. In these cats, repeating the test in a week or so is often diagnostic. If not, more specialized testing must be done, such as T3 (another thyroid hormone) suppression tests or thyroid releasing hormone (TRH) testing.
There are currently three commonly used treatments for this problem. Surgery, radioactive iodine therapy and medical treatment using methimazole (Tapazole (rx)).
Medical treatment may be effective long term if the cat is compliant about taking pills and no side effects occur. It is not uncommon to see side effects from methimazole, which can include anemia and decreased platelet numbers. Most side effects occur within the first month. Medical treatment is less costly in the short run but over the lifetime of the cat, it may be more expensive than alternative methods of treating for this problem.
Surgery is an effective procedure in most cats. There is a higher than normal risk of complications with surgery on the thryoid gland, due to the parathyroid glands in the region. These glands control calcium regulation in the body and they are easily damaged during surgery. Death can result if calcium levels drop sufficiently. Therefore, calcium levels should be carefully monitored for a week if both of the thyroid glands are affected.
Radioactive iodine therapy is probably the best combination of safety and efficiency for treatment of hyperthyroidism. It is effective in about 90% of cats, no surgery or anesthesia is required and the parathyroid glands are not affected. For patients in which the daily administration of pills is undesirable this is the best method of treatment. The major disadvantage to this therapy is the required isolation of the cat at a treatment facility for 7 to 14 days following the administration of the radioactive iodine, due to safety concerns.
In older cats, this disease is common enough that routine screening is considered to be necessary by many veterinarians. Due to the potential for numerous secondary complications, such as heart disease and digestive problems, early diagnosis is a good idea. Consider asking your vet about tests for hyperthyroidism if your cat is over 10 years of age.
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Last edited 01/13/08