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What is Hyperthyroidism?

Hyperthyroidism is characterized by the overproduction of thyroid hormone and a subsequent increase in metabolic rate. This disease is common in older cats. Although the thyroid gland enlarges in hyperthyroidism, it is usually a benign or non-malignant change. Fewer than 2% of hyperthyroid cases in cats involve malignant thyroid gland tumors. The exact cause of hyperthyroidism has not been identified, although the role of dietary iodine continues to be investigated as an influencing factor in susceptible cats.

What are the clinical signs?

The typical cat with hyperthyroidism is middle aged or older – the average age of affected cats is approximately 12 years. Only about 5% of hyperthyroid cats are younger than 10 years of age.

The most common clinical sign of hyperthyroidism is weight loss due to the increased rate of metabolism and despite an increased appetite. Affected cats are often restless, and may become cranky or aggressive. They may have increased water consumption and urination. It is also common for hyperthyroid cats to exhibit increased vocalizing, particularly at night. They may develop periodic vomiting or diarrhea, and fur may appear unkept. In some cats, anorexia develops as the disease progresses.

Two secondary complications of hyperthyroidism can be significant; Hypertension (high blood pressure) and a particular form of heart disease called thyrotoxic cardiomyopathy. Hypertension develops due to the increased pumping pressure and elevated heart rate that occurs with thyrotoxic cardiomyopathy. About 25% of cats with hyperthyroidism become hypertensive. In some cats, blood pressure can become so high that retinal bleeding or retinal detachment will occur, resulting in sudden blindness. Thyrotoxic cardiomyopathy may develop because the heart enlarges and thickens 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 occur.

How is it diagnosed?

Diagnosis of feline hyperthyroidism is generally 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 T4 by Equilibrium Dialysis (FT4 by ED) or a T3Suppression 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.

How is it treated?

Since fewer than 2% of cats with hyperthyroidism have cancerous growths of the thyroid gland, treatment is usually very successful. The most common treatment for hyperthyroidism is the medication Methimazole. Life-long administration of Methimazole can control the effects of an overactive thyroid gland. It takes several weeks for Methimazole to restore thyroid hormone levels to normal. Methimazole blocks the production of excess thyroid hormone rather than destroying the abnormal thyroid tissue, so the drug must be given for the remainder of the cat’s life. After one month of treatment, a blood sample is taken to verify that the thyroid hormone levels are normal. All other treatment options will be reviewed by your veterinarian at time of diagnosis.

What is the prognosis?

The outcomes following most hyperthyroid therapies are usually excellent, and most cats have a very good chance of returning to a normal state of health. Cats managed with diet or medication generally do well as long as their feeding is consistent or their medication is administered routinely and follow-up blood and diagnostic test schedules are performed. Ideally, T4 levels should be tested every six months.

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