MEASUREMENT OF TOTAL AND UNBOUND TESTOSTERONE
In healthy women, approximately, 50 to 60 percent of testosterone is bound to sex hormone binding globulin (SHBG), 30 to 40 percent to albumin and only 0.5 to 3 percent is unbound. The free hormone
hypothesis assumes that only the free and loosely bound (testosterone bound to albumin) exerts
SHBG is affected by many factors; it is increased by thyroid hormone, estrogen
and aging, and decreased by testosterone, glucocorticoids, growth hormone and insulin. Ideally,
measurements of unbound testosterone should provide a better estimate of the biologically
relevant fraction. However, in practice, some measurements of free and bioavailable testosterone
have been hampered by methodological difficulties, particularly in women.
The commercially available assays for the measurement of unbound testosterone include the equilibrium dialysis method, the bioavailable testosterone by the ammonium sulfate precipitation method, free testosterone indices calculated from the measured total testosterone and SHBG concentrations, and tracer analog methods for estimates of free testosterone.
Of these methods, the equilibrium dialysis method for the measurement of unbound testosterone, and ammonium sulfate precipitation method for the measurement of albumin-bound and unbound testosterone (bioavailable testosterone) are both acceptable methods that have good clinical correlation, are accurate, independent from the effects of SHBG concentrations, and available at specialized commercial endocrinology laboratories.
The estimates of free testosterone, calculated from total testosterone and SHBG concentrations, have also been shown to correlate with free testosterone concentrations measured by dialysis in men. These algorithms have not been extensively tested in women. Tracer analog methods of measuring free testosterone are widely available but are affected by SHBG concentrations and do not provide an accurate measure of unbound testosterone and are not recommended.