The thyroid glands are located in the neck and play a vital role in regulating the body's metabolic rate. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in the metabolic rate. This is a common disease of older cats. Although the thyroid gland enlarges in hyperthyroidism, it is usually a benign or non-malignant change. Less than 2% of hyperthyroid cases involve malignant thyroid gland tumors.
Many organs are affected by hyperthyroidism, including the heart. The heart is stimulated to pump faster and more forcefully; eventually, the heart enlarges to meet these increased demands for blood flow. The increased pumping pressure leads to a greater output of blood and subsequent high blood pressure. About 25% of cats with hyperthyroidism have high blood pressure.
Are certain cats more likely to develop hyperthyroidism?
Older cats are at increased risk for developing hyperthyroidism. Environmental and dietary risk factors have been investigated. They may predispose some cats to hyperthyroidism, although the specific mechanisms are not known. No individual breed is known to be at increased risk, although the Siamese appears to have a somewhat increased incidence of hyperthyroidism compared to other breeds.
What are the clinical signs of hyperthyroidism?
"The typical cat with hyperthyroidism is middle aged or older."
The typical cat with hyperthyroidism is middle aged or older. The average age of affected cats is approximately twelve years. The most common clinical sign of hyperthyroidism is weight loss secondary to the increased rate of metabolism. The cat tries to compensate for this with an increased appetite. In fact, some of these cats have a ravenous appetite and will literally eat anything in sight! Despite the increased intake of food, most cats continue to lose weight. The weight loss may be so gradual that some owners will not realize it has occurred, or the weight loss may be quite rapid. Affected cats often have increased water consumption and urination. There may be periodic vomiting or diarrhea, and the fur may appear unkempt. In some cats, anorexia develops as the disease progresses.
Two secondary complications of hyperthyroidism can be significant. These include hypertension (high blood pressure) and heart disease called thyrotoxic cardiomyopathy. Hypertension develops due to the increased pumping pressure and elevated heart rate that occurs with thyrotoxic cardiomyopathy. In some cats, blood pressure can become so high that retinal hemorrhage or retinal detachment will occur and result in sudden blindness.
"Both the cardiomyopathy and the hypertension are potentially reversible..."
Heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands. In some cases, the cat will develop a heart murmur associated with the cardiomyopathy. Both the cardiomyopathy and the hypertension are potentially reversible with appropriate treatment of the disease. However, unless the retinal detachment is treated immediately, permanent blindness can result.
What causes hyperthyroidism?
Some of the risk factors for hyperthyroidism have been defined above. A specific cause for hyperthyroidism has not been identified. The possible role of dietary iodine continues to be investigated as a dietary influence on development of hyperthyroidism.
How is hyperthyroidism diagnosed?
"In most instances, diagnosis of this disease is relatively straightforward."
In most instances, diagnosis of this disease is relatively straightforward. The first step is to determine the blood level of one of the thyroid hormones, called total thyroxine (or TT4). Usually, the TT4 level is so high that there is no question as to the diagnosis. Occasionally, a cat that is suspected of having hyperthyroidism has a TT4 level within the upper range of normal. When this occurs, a second test, usually either a Free T4by Equilibrium Dialysis (FT4 by ED) or a T3 Suppression Test is performed. If these tests are not diagnostic, a thyroid scan can be performed at a veterinary referral center or the TT4 can be measured again in a few weeks.