Estimation of free thyroid hormone concentrations in the clinical laboratory.

A multiplicity of tests have come into routine use for the estimation of the concentrations of free thyroid hormones in serum and plasma since the introduction, almost three decades ago, of the hypothesis that only the free thyroid hormone is metabolically active. Most of these tests give estimates of considerable diagnostic value, although results may be unreliable in some clinical situations. This review outlines the methods currently available for estimating free thyroid hormones and highlights the extrathyroidal and physiological factors influencing the results of such tests, particularly for the analog-based techniques.