Why can total testosterone look normal while free testosterone is low, and what role does SHBG play?

Total testosterone includes both unbound hormone and testosterone carried by proteins, principally SHBG and albumin. When SHBG is high, the total can appear normal while the free fraction is relatively low. When SHBG is low, a low total result may coexist with a less reduced free concentration. This is why total testosterone, free testosterone and SHBG need to be interpreted together in selected cases rather than ranked as competing scores. Assay accuracy and symptoms also matter.[1]

The Endocrine Society recommends reliable free-testosterone assessment when total testosterone is near the lower limit or a condition affecting SHBG makes interpretation difficult. Equilibrium dialysis or an appropriate calculation using total testosterone, SHBG and albumin is preferred over unreliable direct analog assays. Results from different methods and laboratories are not automatically interchangeable. In someone already using testosterone, the timing relative to administration adds another variable. A discordant result is a reason to check the measurement and context, not automatically increase the dose or try to suppress SHBG.[1]