ATA Guidelines Tools

3 Interpreting Thyroid Tests Table 1. Key Clinical Points for Interpreting Thyroid Tests • Nowadays, most thyroid tests are made on multianalyte automated immunoassay instrument systems. • The same test reported by different manufacturer instruments can differ in absolute value and requires reference intervals appropriate for the instrument and population for optimal interpretation. • Physicians may not be aware of changes in laboratory instruments and refence intervals. When in doubt, physicians may discuss such issues with the laboratory. • Interfering factors in thyroid hormone tests are rare but can affect any thyroid test result. Interfering factors include heterophile antibodies (HAbs), analyte autoantibodies, high-dose dietary biotin, pharmaceuticals, and nutritional factors. • HAb interference can affect multiple tests. When HAb interference with a test is detected, it should be noted in the patients' chart since it may also affect other laboratory investigations. • Interference should be suspected when the test result is discordant with the clinical presentation of the patient. When suspected, the physician may discuss the case with the laboratory for further evaluation. • The reference interval for a test typically represents the 95% confidence limits for a reference population without thyroid disease. • The between-subject biological variation is wider than the within-subject biological variation and the thyroid test result (particularly TSH) may need to be significantly abnormal for an individual to exceed the reference intervals. • TSH represents a more sensitive reflection of thyroid status than f T4, because of the log-linear TSH–f T4 relationship. However, TSH is only a valid measure of thyroid status if the hypothalamic-pituitary axis is intact. • It may take weeks to months for the TSH measurement to accurately reflect thyroid status after an acute change (e.g. change in medication).

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