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*The measurement of T4 and T3 was complicated by the large influence of binding proteins. This led to the ‘free hormone index’ where the total TH concentrations were mathematically corrected for their binding protein concentration. In the 1980s, direct methods to measure fT4 were developed to overcome the influence of binding proteins. The technique of equilibrium dialysis (ED) or ultrafiltration that separated free and bound T4 were combined with the measurement of this (f)T4 using RIA. Later, this last step was replaced by liquid chromatography tandem-mass spectrometry (LC-MS/MS) measurement and equilibrium dialysis followed by this LC-MS/MS is now considered as the gold standard 14. However, it was, and still is, a challenge to directly measure fT4 reliably, due to its concentration in the picomolar range and the subtle equilibrium between free and bound T4. The technique of equilibrium dialysis followed by RIA or LC-MS/MS is an expensive, time consuming, and labor intensive technique which makes it unsuitable to use as routine test in clinical laboratories