Free testosterone
The small unbound fraction available to tissues — often the number that matters.
Free testosterone is the fraction circulating unbound to SHBG or albumin — a small share of the total, but the portion most readily available to tissues. It is either measured directly by specialist assays or calculated from total testosterone, SHBG and albumin.
It matters because clinical guidelines advise measuring it when SHBG may be altered — in obesity, ageing, thyroid disease or diabetes — since total testosterone alone can then read as normal while the available fraction is low, or vice versa.
In practice, testosterone is measured in the morning when it peaks, confirmed on a repeat sample, and interpreted with symptoms: one number in isolation does not make a diagnosis, and diagnosis is a clinician's job.
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Written and reviewed by Mathew Beale, MSc Biotechnology, University of Reading.
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