Testosterone Free, Profile I

Free testosterone is the concentration of unbound testosterone in serum.¹ The majority (approximately 60% to 90%) of serum total testosterone is associated with sex hormone binding globulin (SHBG); this fraction is tightly bound and biologically unavailable to its target tissues. The remaining bioavailable testosterone is mostly bound to albumin, with only a small fraction (approximately 0.5% to 2%) circulating in the free form.^1,2 Free testosterone is the form of testosterone that can diffuse into the tissues and act on receptors and is considered the active fraction by many physicians.^1,3,4 Free testosterone assessment is recommended as a primary or secondary measure of androgen activity in men^2,5-11 and women.^12-16 The equilibrium dialysis or ultrafiltration methods are recommended, however they are relatively difficult and expensive to perform. All of these methods are currently available from LabCorp.

The primary screening test for the diagnosis of hypoandrogenism in men is the measurement of total testosterone in serum in a morning sample.^2,5-11 Low concentrations of testosterone in serum should be confirmed by repeat measurement, preferably using liquid chromatography/mass spectrometry.^6,7,10 Determination of free testosterone can be of value in men with borderline total testosterone because alterations in SHBG levels can markedly affect the concentration of biologically available free testosterone.^2,17-20 Factors and conditions that tend to increase SHBG concentrations in men include aging, hyperthyroidism, estrogens, HIV disease, anticonvulsant therapy, and liver disease.² Factors and conditions that tend to decrease SHBG concentrations in men include obesity, diabetes mellitus, hypothyroidism, glucocorticoid therapy, androgenic steroid therapy, nephrotic syndrome and acromegaly.²

The binding affinities between testosterone, SHBG and albumin have been measured experimentally, and it has been shown that if the concentrations of serum total testosterone, SHBG, and albumin are measured, the corresponding concentration of free testosterone can be accurately estimated according to equilibrium kinetics.²¹ The widely accepted method of calculation of serum free testosterone is based upon a 1999 paper by Vermeulen.²¹ The authors derived the equation based upon the law of mass action. Using the known association constants between testosterone, SHBG, and albumin, they calculated the theoretically predicted serum free testosterone concentrations. This method demonstrated substantial agreement with equilibrium dialysis measured concentrations when testosterone, SHBG and albumin are measured using reliable assays.