It has been demonstrated that the calculated free testosterone derived using commonly available online calculators, which are based on simpler models assuming simple 1:1 binding of testosterone and SHBG [1,2], deviate from measurements obtained by equilibrium dialysis by as much as 40% [3-9].
Current methods for measuring free testosterone (fT) are technically challenging and not accurate. The widely used direct immunoassay and tracer analog techniques for measuring fT have been shown to be inaccurate. Equilibrium dialysis, the reference method against which other methods are compared, is labor-intensive and cumbersome, and therefore has had limited clinical adoption. Recently, Endocrine Society’s Expert Panel acknowledged the experimental problems in fT measurements and concluded that "...the calculation of free testosterone is the most useful estimate of free testosterone in plasma..." For this reason they advocate for indirect "calculator" based methods, where free testosterone can be computed from the total testosterone, SHBG, and albumin concentrations.However, we have demonstrated that even the calculated fT values derived from the prevailing equations, based on linear law-of-mass action models or empiric equations, differ systematically from free testosterone measured by equilibrium dialysis by as much as 40%.
Recent biophysical studies have established that SHBG is a dimer that exhibits allostery in its interaction with testosterone. This consideration allows TruT™ to more accurately model free circulating testosterone .
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