[Clinical application of the biological variability data of the thyroid hormones]. / Aplicación clínica de los datos de variabilidad biológica de las hormonas tiroideas.

BACKGROUND: The selection of laboratory tests with greater effectiveness for identifying patients with thyroid disorders (sensitivity) from non affected subjects (specificity) is, at present, under discussion. The study of biological variation can contribute to point out the laboratory tests with greater effectiveness. METHODS: Total blood was collected daily for five consecutive days from 25 healthy volunteers, which maintaining their usual life style all over the period. RESULTS: The biological coefficients of variation--within and between--subject, the individuality indexes and the critical differences between consecutive results are estimated. The effect of pulsatility over the results obtained is also discussed. CONCLUSIONS: The most effective laboratory test for the diagnosis of thyroid disorders is TSH, with the precaution of doing a second analysis if the first result falls close to either the lower or higher reference range limits. The best laboratory tests for the monitoring of hypo- and hyperthyroidism are total T4 and free T4, in which a critical difference between consecutive results higher than 15% indicates a significant change in the patient status.