Timely diagnosis and treatment of thyroid dysfunction is compelling given the prevalence and severity of the disease. It requires reliance on adequate laboratory testing of serum TSH as a hallmark in combination with free thyroxine/triiodothyronine. Free hormone methods have to accommodate variations in the concentration and binding capacity of binding proteins. This is a challenge because none of the methodologies developed so far measures the actual unbound hormone in serum. The indirect methods provide an approximation while the direct ones estimate the free hormone concentration either in the presence of the protein-bound counterpart, or after physical separation of the free from bound fraction. The ongoing controversy on the validity and lack of comparability of methodologies points to their imperfectness to reflect real in-vivo free hormone concentrations. Therefore, laboratories and clinicians should know the window of validity and limitations of their methods. The recently developed reference measurement system is a key advance towards improved standardization and clinical validity of free thyroid hormone measurements.
Keywords: T3; T4; TSH; equilibrium dialysis; free T4 index; international conventional reference measurement procedure; isotope dilution-liquid chromatography-tandem mass spectrometry; one-step labeled analog assay; one-step labeled antibody assay; thyrotropin; thyroxine; tri-iodothyronine; two-step labeled analog assay; ultrafiltration.
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