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Testing for Hyperthyroidism

Clinician's Brief (Capsule)

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Increased routine screening for hyperthyroidism has increased potential for false-negative and false-positive results in asymptomatic cats. This study reviewed common thyroid function tests, using tests from 100 hyperthyroid cats to evaluate results. For any thyroid function evaluation, test results should be evaluated in conjunction with signs, history, examination findings, and other laboratory findings.

Palpation of a cervical thyroid nodule is a highly sensitive but poorly specific test. Total thyroxine (T4) levels provide a good screening test, particularly in cats with supportive signs, as over 90% of hyperthyroid cats have high levels. Human T4 kits are not recommended. Radioimmunoassay (RIA) is the gold standard, but chemiluminescent enzyme immunoassays (CEIA) provide similar results. In-house ELISA results (less reliable) overestimate T4 levels. Enzyme immunoassay (EIA) correlates well with RIA results but have a higher rate of false-positives and false-negatives than CEIA. Up to one-third of hyperthyroid cats can have normal T3 levels, so this test is not helpful in supporting diagnosis. Hyperthyroid cats can have normal T4 and T3 levels for several reasons, including early or subclinical disease, fluctuating T4 levels, or severe nonthyroid illness. Retesting T4 levels after addressing concurrent disease, evaluating via another method (RIA or CEIA), or considering other thyroid function testing (eg, free T4, TSH, T3 suppression test, thyroid scintigraphy) was recommended. Free T4 by equilibrium dialysis, when interpreted with T4 levels, is useful as it is more sensitive than T4 but with poor specificity.

Commentary

Feline hyperthyroidism is an example of a disease caused by factors which, if specifically identified, could be altered to potentially eradicate this disease. This review emphasized a few major points. The first is that feline hyperthyroidism was virtually absent before 1979 and is therefore likely caused by factors to which the feline population has been exposed since. Also, pathogenesis of hyperthyroidism is likely multifactorial, and steps have been taken to identify individual dietary and environmental factors involved in the disease process. Through further research and intervention, there is potential to prevent hyperthyroidism from affecting the future feline population.—Jennifer Ginn, DVM, DACVIM

Source

More than just T4: Diagnostic testing for hyperthyroidism in cats. Peterson ME. J FELINE MED SURG 15:765-777, 2013.

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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