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Hyperthyroidism Hyperthyroidism is the most common endocrine disorder of cats. It is caused by excess production of thyroxine by the thyroid gland. In most cases this is the result of a benign tumour of the thyroid gland, about 1-2% of cases however will have the more malignant thyroid carcinomas. The thyroid gland is not palpable in the normal animal. In hyperthyroidism enlargement may occur. This enlargement is palpable in most affected cats. Hyperthyroidism occurs in older cats but has been recorded at 4 years of age, 95% of affected cats are over 10 years old. Common clinical signs include weight loss, increased appetite, increased thirst, vomiting, increased activity, an diarrhea. Occasionally cats will present with lethargy, depression anorexia, and/or weakness. This is known as apathetic hyperthyroidism and occurs in 5-10% of affected cats. Breathing difficulties and heart failure are also infrequently seen. The main differential diagnoses for hyperthyroidism include diseases causing increased appetite with simultaneous weight loss (diabetes mellitus, inflammatory bowel disease, alimentary lymphoma, and, uncommonly, chronic renal failure, intestinal parasitism, and exocrine pancreatic insufficiency) and diseases causing increased thirst and urination (diabetes mellitus, chronic kidney failure, and liver disease). Consequences of untreated hyperthyroidism include heart disease and raised blood pressure. Diagnosis and management of raised blood pressure is important hyperthyroid cats; however, high blood pressure generally resolves following successful treatment of hyperthyroidism. Around 90% of affected cats have an elevation of liver enzymes detectable on blood testing, many also show raised levels of red blood cells. Blood thyroxine levels are elevated in around 90-98% of cats and this is the most commonly used test to diagnose this problem. Concurrent disease may cause normal thyroxine levels in hyperthyroid cats and in these cases further blood testing may be necessary to achieve diagnosis. Blood pressure monitoring should be carried out. Treatment is with a drug called neomercazole which inhibits the synthesis of thyroxine by the thyroid gland. Other alternatives are surgery to remove one or both of the thyroid glands or the use of radioactive iodine to destroy abnormal thyroid tissue. The decision on which option to use is best discussed with the vet on an individual case basis. Treatment to reduce blood pressure and control any concurrent cardiac disease should be considered. Most cats with hyperthyroidism can be successfully treated. Cats with concurrent kidney or heart failure have the poorest prognosis. |
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