Clinical evaluation of free thyroxine calculation from thyroxine and thyroxine binding globulin radioimmunoassays

Free T4 calculation was performed based on the equation of the law of mass action between T4 and TBG assessed by routine radioimmunoassay. To examine the validity of this calculation, sera from normal subjects, from hypo- and hyperthyroid patients, from subjects with hereditary deficiency or increase in TBG, and those from uncomplicated pregnancy were included in the assay and the results were compared with those directly measured by radioimmunoassay with encapsulated antibody. Free T4 calculation on the sera described above was almost equal to the free T4 concentration measured directly and reflected clinical thyroid states. Thus, the proposed equation for the calculation of free T4 from total T4 and TBG radioimmunoassay is useful for clinical purposes as well as direct free T4 radioimmunoassay.