Pitfalls to Avoid While Interpreting Thyroid Function Tests: Five Illustrative Cases

Michael J. Fowler, MD, Aaron F. Pannone, BA, Lewis S. Blevins, Jr., MD

Disclosures

South Med J. 2002;95(5) 

In This Article

Introduction

Thyroid disorders occur in 5% to 10% of the population. As a result, laboratory tests of thyroid function are among the more common tests ordered in clinical practice. Treating physicians have at their disposal a large number of tests from which to choose in evaluating patients with suspected thyroid dysfunction ( Table 1 ). In this era of cost consciousness, it is important not only to select tests that will provide the most information regarding thyroid function, but also to interpret the results accurately.

Several important points regarding laboratory assessments of thyroid function are worth mentioning. Test results must be interpreted in conjunction with a clear understanding of the pathophysiology and natural history of the suspected disorders. While clear patterns of test abnormalities are associated with certain disease states, many disorders are not static (eg, postpartum thyroiditis). One must keep in mind that laboratory tests only represent a snapshot of thyroid function at the time that the blood sample was obtained. Thyroid function test abnormalities do not always reflect thyroid dysfunction. A number of drugs and disorders affect thyroid hormone transport, metabolism, and the radioimmunoassays used to determine hormone concentrations. In many of these situations, patients are demonstrably euthyroid, despite obvious test abnormalities. Conversely, some test results may be normal in the setting of real thyroid dysfunction (eg, subclinical hyperthyroidism and hypothyroidism). Additional tests are often necessary to ascertain the economy of thyroid function and to determine if normal results reflect any deviation from the patient's true set-point of thyroid function. It is important to recognize that, in most cases, normal ranges of thyroid function tests are established by assessing a reference population and setting the reference range limits at ± 2 standard deviations from the mean. Reference ranges vary depending on age, laboratory, and methodology.[1]

A comprehensive review of all aspects of thyroid function tests and their interpretation is beyond the scope of this manuscript. Therefore, we will use exemplary cases referred to us in the endocrinology clinics for evaluation. In each case, errors in interpretation of thyroid function tests were made, and these will serve to initiate relevant discussion.

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