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Hyperthyroidism in Cats




Overview
Contributing Factors
Clinical Signs
Diagnosis
Treatment Options
Prognosis
Prevention

Overview
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The thyroid gland, located inside the lower neck, plays an important role in regulating the body's rate of metabolism. Hyperthyroidism is a disorder characterized by the enlargement of the thyroid gland, overproduction of thyroid hormone, and a subsequent increase in a cat's metabolic rate. This condition is fairly common in senior cats. Most cases are benign; less than 1% involve a thyroid gland malignancy.

Many organs are affected by hyperthyroidism, including the heart. The disease stimulates the heart to pump faster and more forcefully, and eventually the heart enlarges to meet these demands. The increased output of blood from the heart may lead to high blood pressure and, in some cases, heart failure. The liver is also susceptible to damage by overproduction of thyroid hormone, as are the kidneys and gastrointestinal tract.

Contributing Factors
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Aging is the main factor contributing to the development of hyperthyroidism. Environment and diet have been investigated and may play a role in predisposing cats to hyperthyroidism, though the specific risk factors and mechanisms are unknown. No breed is known to be at increased risk.

Clinical Signs
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The typical cat with hyperthyroidism is middle-aged or older. The most consistent finding among cats with this disorder is weight loss secondary to the increased rate of metabolism. Many cats compensate for this loss by eating more than usual; some hyperthyroid cats may develop a ravenous appetite. Despite the increased intake of food, most cats gradually lose weight. The weight loss can be so gradual that some owners do not notice it until their cat has lost 10-20% of his body weight. Affected cats may also drink more water and pass larger volumes of urine than they did prior to developing this condition. Other physical symptoms include periodic vomiting, soft stool or diarrhea, and an unkempt hair coat. Some cats develop anorexia as the disease progresses. Behavioral symptoms include elimination outside the litter box, increased irritability, increased activity levels, and other changes.

Two secondary complications of this disease can be significant. They include hypertension (high blood pressure) and a heart disease called thyrotoxic cardiomyopathy. Hypertension develops as a consequence of the pressure exerted on the arteries by the increased output of blood from the heart. Some cats' blood pressure becomes so elevated that strokes occur. Severe hypertension may also cause retinal hemorrhage or detachment and sudden blindness. Thyrotoxic cardiomyopathy occurs when the heart muscle stretches and thickens due to excess production of thyroid hormone; as a result, the heart pumps less efficiently. Both hypertension and thyrotoxic cardiomyopathy are reversible with appropriate treatment, especially if the conditions are diagnosed at an early stage.

Diagnosis
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In most instances, diagnosis of this disease is relatively straightforward.

  • Enlargement of one or both thyroid lobes is one of the first changes to occur. The thyroid glands are too small to be easily palpated (felt) during a physical examination of a non-hyperthyroid cat. If the glands can be palpated, the cat likely has hyperthyroidism.
  • The first blood test performed to diagnose hyperthyroidism measures the level of thyroxine (T4), one of the thyroid hormones. A T4 level elevated beyond the normal reference range usually confirms the diagnosis.
  • If a cat is suspected of having hyperthyroidism (i.e., has a T4 level within the upper range of normal limits), a second test, called a Free T4, is performed on the same blood sample to help confirm the diagnosis.
  • If neither of these tests is diagnostic and hyperthyroidism is still suspected, a thyroid scan can be performed at a veterinary referral center. Alternatively, because T4 levels wax and wane, the T4 test may be repeated two to three weeks later.

Treatment Options
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Because less than 1% of hyperthyroid cats have cancerous growths of the thyroid gland, treatment is usually successful. Treatment of cats with cancerous thyroid nodules also may be successful, however. Options include medical management (usually oral medication), radioactive iodine therapy, dietary management, and surgical removal of the affected thyroid gland(s). The best therapy for a cat depends on many factors.

  • Medical management: In most cases, initial treatment involves medical management. Administration of an oral drug, usually Felimazole (methimazole), can blunt thyroid production and thus control the effects of the overactive thyroid gland. Some cats (less than 10%) react adversely to the drug. Side effects, including vomiting, lethargy, anorexia, fever, and anemia, may begin to occur as late as several weeks after initial administration. If methimazole causes any of these side effects, one of two alternative medications can be prescribed. Most cats tolerate at least one of the three medications. Methimazole does not destroy the abnormal thyroid tissue, so the drug must be given for the remainder of a cat's life if not used in conjunction with other therapy. Long-term medical management is an acceptable option as long as the drug is well tolerated by the cat and the cat's thyroid levels are adequately controlled with regular administration. Oral medication must be administered daily (usually twice per day), and the dosage usually needs to be adjusted periodically to maintain normal thyroid hormone levels. Methimazole typically is dispensed in tablet form, but alternative forms are available to owners who are unable to administer pills to their cats (e.g., transdermal gels, chews, liquids). Blood work is generally done every three to six months to monitor thyroid, liver, and kidney values but may need to be done more frequently if the thyroid levels are not well regulated or if a cat's kidney function is deteriorating. In its early states, hyperthyroidism may mask declining kidney function because the hypertension that often accompanies hyperthyroidism increases blood flow to the kidneys. When a cat with underlying kidney insufficiency is treated for hyperthyroidism/hypertension, renal blood flow returns to normal and the kidney insufficiency becomes more apparent. Conversely, hyperthyroidism left untreated can have a deleterious effect on the kidneys as well. When long-term medical management is not the best option for a cat, two permanent corrective procedures are possible once the thyroid level is well regulated and kidney function is stable: (1) radioactive iodine treatment and (2) surgical thyroidectomy. These options may be considered with or without a preliminary medical management (oral medication) trial.

  • Radioactive iodine: The most effective and least invasive way to destroy all of the abnormal thyroid tissue is radioactive iodine therapy. It is almost always curative and may even eradicate malignant thyroid tumors. Veterinary clinics must be licensed to administer radiation therapy. After treatment, patients must be hospitalized for two to five days. For younger senior cats (those between ages 8 and 12) with normal kidney function at the time hyperthyroidism is diagnosed, we often recommend radioactive iodine therapy rather than a medication trial.
  • Dietary management: In late 2011, a prescription veterinary diet was introduced for the treatment of hyperthyroidism in cats. Although the exact cause of hyperthyroidism has not been identified, a diet low in iodine has shown potential as a means of reducing elevated thyroid levels.
  • Surgery: The fourth option is surgical removal of the affected thyroid lobe(s). Because hyperthyroid cats are usually older than age 8 and because heart and metabolic abnormalities are associated with hyperthyroidism, a degree of risk is involved. Surgery is an invasive procedure, and postoperative complications may arise. If ectopic thyroid tissue (tissue in abnormal locations) is not detected and therefore not removed during surgery, hyperthyroidism may redevelop. Now that radioactive iodine therapy is available in the Chicago area, we rarely recommend this surgery.

Prognosis
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Many owners of geriatric cats with hyperthyroidism hesitate to choose radiation therapy because they believe advanced age will negatively affect the results. However, old age is not a disease. The outcome following both medical management and radiation therapy is usually positive, and most cats have a good chance of returning to an excellent state of health for many years.

Prevention
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No preventive measures are known, but middle-aged and geriatric cats should have a complete physical examination by a veterinarian every 6-12 months.



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