PREANALYTICAL CONSIDERATIONS IN TESTING THYROID-FUNCTION

Remarkable technical advances have permitted analytical measurement of thyrotropin (TSH) and estimates of free thyroxine (ET(4)) with precis ion, accuracy, and favorable economics. Combined with an increased app reciation of the key insights into the pituitary-thyroid relation, pre analytical considerations infrequently introduce confounding variables . In reviewing thyroid data, preanalytical considerations include phys iological and specimen-based issues. Central to the improvement in thy roid assessment is the recognition that physiological individuals main tain their FT4 within narrow limits. When this deviates, there is a lo garithmic response of the TSH concentration to the arithmetic shift in FT4. In effect, the TSH deviation magnifies the subtle shift in FT4. Artifact and other nonthyroid-related preanalytical considerations are infrequently the cause of nonconcordance when discrepancy occurs betw een the reported values for FT4 and TSH. When abnormalities of TSH and FT4 are encountered, the probability strongly favors a disease state rather than a preanalytical variable. Infrequent but real extrathyroid al pathophysiological states are increasingly recognized as a result o f the reliable assessment of the pituitary-thyroid relation.