Free testosterone
The small unbound fraction available to tissues: often the number that matters.
Free testosterone is the small fraction of circulating testosterone not bound to a carrier protein, and therefore available to enter cells and act. Typically only around 1 to 3% of total testosterone is free, with most bound tightly to SHBG and a further portion bound loosely to albumin.
Bioavailable testosterone is a related term, covering the free fraction plus the albumin bound portion, on the reasoning that the albumin bond is weak enough to release hormone at the tissue.
It matters because total testosterone can mislead in either direction. Anything that changes SHBG changes the relationship between total and free, so obesity, insulin resistance, thyroid disease, ageing and some medications can all produce a total result that does not reflect what tissues are actually receiving.
Measuring it well is harder than measuring the total. Equilibrium dialysis is the reference method and is not universally available; calculated free testosterone from total, SHBG and albumin is the common practical approach; and the direct immunoassays sold by some laboratories are considered unreliable.
Clinical guidance therefore reserves it for specific situations: when total testosterone is near the lower limit of normal, or when a condition affecting SHBG is present, rather than as a routine first test.
It is also the number most enthusiastically quoted by supplements and clinics, precisely because it is harder to check and easier to move on paper by changing the method.
In practice, testosterone should be assessed on repeated morning samples with SHBG alongside, and interpreted by a clinician rather than against a chart from a testing company.