Protein bound testosterone is bound to:
(1) SHBG (largest percent)
(2) albumin (most of non-SHBG bound testosterone)
(3) cortisol-binding globulin (very small amount)
Equation from Nanjee and Wheeler (1985), Figure 1, page 388:
percent free testosterone =
= 6.11 – (2.38 * (LOG10 (SHBG concentration in nmol/L)))
Equation from Anderson (1997):
percent free testosterone =
= 2.28 – (1.38 * (LOG10 (SHBG concentration)))
where:
• In Anderson et al (1975), SHBG is expressed in "mol/L * 10^(-8)", or "10 * nmol/L".
• If this value is used, results differ from that given by the other 2 equations.
• If the SHBG concentration is expressed as µg/dL (equal to nmol/L divided by 34.7), then the results are similar to the other 2 equations.
Equation from Wheeler (1995), Figure 4, page 32:
percent free testosterone =
= 5.56235 – (2.01822 * (LOG10 (SHBG concentration in nmol/L)))
Limitations:
• These equations give a rough estimate of the free testosterone concentration.
• More complex equations that include binding with albumin and cortisol-binding globulin may give a more accurate estimate.
