Free Testosterone

Free testosterone measures the small but mighty fraction of testosterone that circulates in your bloodstream without being attached to any carrier proteins. While total testosterone tells you how much of the hormone is present overall, only about one to three per cent of it is truly free, unbound and ready to enter your cells and do its work. This makes free testosterone an especially useful marker, as it reflects the amount of the hormone that is actively available to your muscles, bones, brain, and other tissues. Think of it this way: if total testosterone is the money in your bank account, free testosterone is the cash in your wallet, it is the portion you can actually spend right now. The rest of your testosterone travels bound to proteins, primarily sex hormone binding globulin and albumin, which act like a storage system, releasing testosterone gradually as your body needs it.

When SHBG levels are high, more testosterone is locked away and less is free; when SHBG is low, more testosterone is available in its active form. This is why free testosterone can sometimes tell a different story from your total testosterone reading, and why clinicians often find it valuable to look at both together.

Several factors influence your free testosterone levels. Because SHBG plays such a central role, anything that affects SHBG will indirectly affect free testosterone, for instance, ageing, body weight, liver health, and thyroid function can all shift the balance. In men, free testosterone tends to decline with age, sometimes more noticeably than total testosterone. In women, changes in how the adrenal glands or ovaries function can influence levels. Lifestyle factors such as exercise, diet, sleep quality, and stress also play a part. If your free testosterone result raises any questions, your GP can interpret it alongside your total testosterone and SHBG to give you a thorough understanding of your hormonal health.

For men, the British Society for Sexual Medicine (BSSM) provides a clear guide for interpreting calculated free testosterone. A level above 0.225 nmol/L (roughly 65 picograms per millilitre) is generally considered sufficient. A level below 0.225 nmol/L suggests testosterone deficiency and is most informative when total testosterone sits in the borderline range of 8 to 12 nmol/L, where free testosterone helps confirm whether the bioavailable fraction is genuinely low. A level above 0.70 nmol/L sits above the typical adult-male range and is worth discussing with your clinician, particularly to consider sample timing or repeat measurement. Morning samples (8-11 am) and a confirmatory repeat are typically used before clinical conclusions are drawn. For women, reference ranges are much lower and are interpreted differently depending on age and life stage. These thresholds are guides, not diagnoses, and your GP will weigh them alongside your total testosterone, SHBG, symptoms, and overall clinical picture.