Free thyroid hormone measurement. A critical appraisal.

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