Diagnosis
Typical clinical signs are polyuria, polydipsia, weight loss, vomiting, and one or more palpable thyroid nodule(s) (Figure 9.1). Diagnosis is confirmed with total thyroxine (TT4) serum measurement.
Weight loss or presence of a palpable thyroid nodule (or nodules) justifies investigation for hyperthyroidism in a geriatric cat. Complete blood count, biochemistry, and urinalysis are essential to eliminate concurrent diseases. A mild to moderate increase in liver enzyme activity is observed in most hyperthyroid patients. An increased serum TT4 is the biochemical hallmark of feline hyperthyroidism and will be observed in most cats (Peterson, Melian, and Nicholls 2001). Measurement of free T4 after equilibrium dialysis (FT4ED) should be used prudently in cats because of its poor specificity in this species.
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