![]() Because of cost, measurement of fT 4 concentration by MED is often reserved for cats with suspected hyperthyroidism where T 4 values are borderline. Measurement of baseline serum fT 4 concentration using the modified equilibrium dialysis technique is a more reliable means of assessing thyroid gland function than measurement of the serum total T 4 concentration, in part, because non-thyroidal illness has more of a suppressive effect on serum total T 4 than fT 4 and serum fT 4 is increased in many cats with occult hyperthyroidism and "normal" T 4 test results. It is important to remember that the thyroid nodule may also be nonfunctional and the clinical signs may be the result of another disease. The T 3 suppression test should only be considered in cats with persistently nondiagnostic serum T 4 and fT 4 concentrations and no access to radionuclide scanning facilities. If available, a radionuclide thyroid scan is preferable over the T 3 suppression test for diagnosing hyperthyroidism in cats with nondiagnostic serum T 4and fT 4concentrations. If the diagnosis is still not established, the veterinarian should consider repeating the serum T 4 and fT 4 tests in 4 to 8 weeks, obtaining a radionuclide thyroid scan, or completing a T 3 suppression test. If the serum T 4 test result is not definitive, the recommendation is to measure serum T 4 and free T 4 (fT 4) in 1 to 2 weeks and to rule out non-thyroidal illness. The diagnosis of hyperthyroidism should not be excluded on the basis of one "normal" test result, especially in a cat with appropriate clinical signs and a palpable mass in the neck. Cats with mild or occult hyperthyroidism and hyperthyroid cats with significant nonthyroidal illness can have "normal" serum T 4 concentrations. Serum T 4 concentrations that fall within the upper half of the normal range create a diagnostic dilemma, especially if clinical signs are suggestive of hyperthyroidism and a nodule is palpable in the ventral region of the neck. An abnormally high serum T 4 concentration strongly supports the diagnosis of hyperthyroidism, especially if appropriate clinical signs are present, and a low serum T 4 concentration rules out hyperthyroidism, except in extremely uncommon situations where severe life-threatening nonthyroidal illness is present. ![]() Measurement of baseline serum thyroxine (T 4) concentrations is extremely reliable in differentiating hyperthyroid cats from those without thyroid disease. The thyroid mass is commonly palpated in the region of the thoracic inlet and may descend into the anterior mediastinum. However, palpation of a cervical mass, by itself, does not confirm and failure to palpate a thyroid mass does not rule out hyperthyroidism. A discrete thyroid mass is palpable in approximately 90% of cats with hyperthyroidism. ![]() ![]() In some cats, lethargy, weakness, and anorexia are the dominant clinical features, in addition to weight loss. Additional clinical signs include haircoat changes, polyuria, polydipsia, vomiting, diarrhea and aggressive behavior. The classic clinical signs include weight loss, polyphagia, and hyperactivity. Hyperthyroidism typically occurs in cats older than 8 years, with a range of 4 to 20 years. The diagnosis of hyperthyroidism is based on identification of appropriate clinical signs, palpation of a thyroid nodule, and documentation of an increased serum T 4 concentration.
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