What is the Origin of Calculated Free Testosterone?

The concept emerged from the recognition that direct measurement of free testosterone is technically challenging and often unreliable in a clinical setting. The mathematical calculation method, such as the Vermeulen equation, was developed to provide a standardized, reliable, and cost-effective surrogate marker for this critical hormone fraction.

What is the Mechanism of Calculated Free Testosterone?

The calculation relies on the principle of mass action, accounting for the strong binding affinity of testosterone to SHBG and the weaker binding to albumin. By inputting the total hormone and binding protein levels, the formula estimates the amount of testosterone that remains free in the plasma, as only this unbound portion can freely diffuse into cells to exert its genomic and non-genomic effects.

Calculated Free Testosterone

Meaning

Calculated Free Testosterone represents an estimated concentration of the biologically active, unbound testosterone in the circulation, which is the fraction capable of interacting with target tissues. This value is mathematically derived using measured concentrations of Total Testosterone, Sex Hormone-Binding Globulin (SHBG), and Albumin, along with established equilibrium constants. Clinical practitioners utilize this calculated value as a more accurate reflection of true androgen status than Total Testosterone alone, particularly when SHBG levels are abnormal.