Veterinary Information: Feline HyperthyroidismPage 3 (5)Differential Diagnoses of Feline Hyperthyroidism: -diabetes mellitus -renal failure -cardiac insufficiency -liver insufficiency -maldigestion / malabsorption -neoplasias Diagnosis: -In 80% of the hyperthyroid cats the thyroid glands can be palpated. These are normally not palpable. (Palpation can take place when the cat’s neck is slightly extended, and the head tilted backwards. Gently pass thumb and index finger through the jugular furrows along both sides of the trachea. Start cranially from the larynx going down the trachea towards the thoracic inlet. Often a movable “nodule” can be felt in feline hyperthyroidism.). -Screening laboratory tests should be done to evaluate general organ functioning and exclude differential diagnoses and renal disease. Note 1: ALT, AST, AP and LDH are high in 50-80% of the cases; high liver enzymes are likely due to a combination of hypoxia, direct thyrotoxic effects, malnutrition and infection. Note 2: Up to 40% of the untreated patients even show an elevated serum level of creatinine and urea nitrogen, indicating underlying renal failure. These patients should be taken into special reconsideration for treatment options. (See Treatment for further information). -Complete blood count: leukocytosis, eosinopenia, lymphopenia are common in hyperthyroid cats and reflects a stress response. Erythrocytosis is also a common finding in hyperthyroid cats (due to a direct stimulation of bone marrow by thyroid hormones as well as an increased production of erythropoietin (EPO) by the kidneys. -Serum T4 concentrations: T4 is high (>100 nmol/l) in the majority of hyperthyroid cats. High-normal values can be found in cats with early symptoms of hyperthyroidism. Note 1: Fluctuations of thyroid hormone levels, or concurrent illness might cause the measured T4 level to be in range while the cat is displaying signs of hyperthyroidism. A cat with a (high-) normal T4 serum concentration with signs of hyperthyroidism should be evaluated with additional tests. Note 2: In middle-aged to old cats with a serious concurrent non-thyroidal illness (e.g. renal failure, hepatic disease, diabetes mellitus) with a high-normal T4 serum concentration, hyperthyroidism should be suspected. Note 3: If T4 is within range, but feline hyperthyroidism is suspected, T4 levels can be measured again after 2 weeks. (Also: rule out other concurrent illnesses).
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