oa CME : Your SA Journal of CPD - Laboratory tests of thyroid function : pitfalls in interpretation - : main article

Volume 24, Issue 7
  • ISSN : 0256-2170



The majority of thyroid disease symptoms are often subtle and nonspecific in presentation. <BR>While clinicians may be alert to the possibility of thyroid dysfunction, they may not always be familiar with the interpretation of laboratory tests, or with the various pre-analytical and analytical factors that can compromise the laboratory tests. <BR>Discordant thyroid test results are often encountered, and an understanding of the TSH/FT4 relationship is required for the selection and accurate interpretation of test results. <BR>The indirect approach (TSH testing) has better diagnostic accuracy in detecting thyroid dysfunction than FT4 testing. <BR>It is more common to encounter misleading FT4 results than misleading serum TSH levels in clinical practice. <BR>The sensitivity of TSH-based strategy is seriously impaired when the blood TSH levels may be normal in the presence of clinically important thyroid disorders, e.g. central hypothyroidism. <BR>Altered thyroid hormone and TSH levels occur in early childhood, in old age and in pregnancy, which may lead to misinterpretation when compared with adult reference range values. <BR>Non-thyroidal factors, such as medications and non-thyroidal illness, can lead to confounding thyroid function test results. <BR>In the case of a non-thyroidal illness, it is preferable to defer testing until the acute illness has resolved, unless there is a strong index of clinical suspicion. <BR>An understanding of the pitfalls of the use of both total (TT4, TT3) and free thyroid hormone (FT4, FT3) use is required for adequate interpretation in certain clinical situations, e.g. pregnancy, and in patients on medications which cause displacement of thyroid hormones from binding proteins. <BR>In addition to the pre-analytical factors, potential analytical factors that interfere with the thyroid function tests assays such as heterophilic antibodies and autoantibodies, may lead to discordant thyroid function test results. <BR>The optimal use of thyroid function tests should be patient-specific and depends on the patient's specific thyroid disease, the stage of the disease and co-existing medical conditions. <BR>Results should be interpreted in the appropriate clinical context of the individual patient with good communication between clinicians and the requesting test laboratory.

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