FREE THYROID HORMONE MEASUREMENT: A Critical Appraisal

摘要: The main purpose of free T4 and T3 assays is to distinguish reliably between thyrotoxicosis, hypothyroidism, the euthyroid state, an objective that cannot be attained with total because hereditary acquired variations in concentrations binding proteins. Effective correction for changes serum concentration TBG can achieved numerous types hormone estimate, but other are not well accommodated. Despite remarkable methodologic ingenuity, no current method reflects undiluted under vivo conditions. Equilibrium dialysis, widely considered reference measurement, also subject error, either preanalytic, owing generation NEFA sample leading overestimate T4, or analytic underestimation effect competitors increase T4. Current approaches measurement vulnerable several method-dependent artifacts: abnormal albumin assay tracer, inhibition by medications, effects critical illness, especially heparin-treated patients, pregnancy, abnormalities sick premature infants. Because systematic variation methods (i.e., whether a technique dependent prone incubation dilution artifacts), it essential consider details evaluating estimates these situations whenever clinically discordant. False-positive more frequent than false-negative results. When results correlated TSH attention assumptions define this relationship, majority false-positive readily identified. If anomaly remains unexplained on repeat sampling, appropriate use alternative depends different principle correlate result authentic measurement.