Every day, your body produces testosterone, a hormone that plays an important role in sexual function, muscle and bone health, red blood cell production and fertility. Although testosterone is often thought of as a male hormone, it is present in both men and women, albeit at much lower concentrations in women.

Most of the testosterone in your bloodstream does not circulate freely. Instead, it is bound to proteins, predominantly sex hormone-binding globulin (SHBG) and albumin. Only a small proportion — approximately 1–4% — circulates as free testosterone.

Testosterone binds strongly to SHBG, meaning that SHBG-bound testosterone has limited availability to tissues. Free testosterone refers specifically to the small proportion that is not bound to either protein.

The amount of SHBG in the blood varies between individuals. This means that two people with the same total testosterone level may have different levels of free testosterone. Measuring or calculating free testosterone can therefore provide additional information in certain circumstances, particularly when SHBG levels are unusually high or low.

Why might total testosterone be normal but free testosterone be low?

Because SHBG binds testosterone strongly, changes in SHBG can affect how much testosterone is circulating freely.

If your SHBG level is high, a greater proportion of your testosterone will be bound. You may therefore have a total testosterone result within the normal range while your free testosterone is relatively low.

The opposite can also happen. If your SHBG level is low, your total testosterone may appear low while your free testosterone remains within the reference range.

This is why free testosterone can provide additional information when total testosterone does not fit with the clinical picture or when SHBG levels are outside the normal range.