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Free T4 and free T3 assay methods typically provide
appropriate values in sera from otherwise well patients with hypothyroidism
or hyperthyroidism, and in sera of euthyroid subjects with moderate alterations
of serum binding protein concentrations (1-3). However, free T4
and free T3 estimates are highly method dependent in sera from
"euthyroid" patients with nonthyroidal illnesses who by definition do not
have concurrent hypothalamic, pituitary or thyroid diseases (Figures 3
and 4) (5,9,17).
Problems with free T4 and free T3 assay methods are multiple and may include: 1) serum dilution, which may weaken effects of low affinity binding inhibitors, 2) sequestration of hormone, 3) artefactual interactions between assay components such as tracers and binding proteins, and 4) alterations of protein binding properties by temperature variations or by buffer pH and composition (9,15,17-19). In addition, in equilibrium dialysis methods, dialyzable contaminating substances in tracer, such as free iodide, can be mistaken for free hormone (19,20), and dialyzable inhibitors of hormone binding to serum carrier proteins may be diluted in dialysate (18). In both equilibrium dialysis and ultrafiltration methods, movement of small quantities of proteins across membranes may increase free hormone estimates (20). Ultrafiltration methods using undiluted serum should avoid in vitro artifacts induced by dilution of serum and by dialyzable inhibitors in serum (18,19). Use of radioimmunoassays which measure free hormone concentrations directly in ultrafiltrate (20), or chromatography to identify the hormone (19) should avoid problems of tracer impurities being mistaken for free hormone (19). |
