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Bioavailable testosterone is the part of the testosterone in your blood that is not bound to the protein SHBG. According to research, this free and loosely bound part usually gives a better picture of your situation than total testosterone.[1]
On this page you can read what bioavailable testosterone is, what the guidelines say about a value that is too low or too high, what affects your value and which values are best to look at alongside it.
Also known as: bio-testosterone, non-SHBG-bound testosterone. At the lab: n/a.
Testosterone travels through your blood. Part of it is tightly bound to SHBG, a protein that binds sex hormones. The rest is free or loosely and non-specifically attached. That free and loosely bound part is called bioavailable testosterone.[1]
In the lab, this part can be measured directly, for example with equilibrium dialysis or by precipitating the SHBG-bound testosterone with ammonium sulphate. These methods are reliable, but they take a lot of time. That is why the value is often calculated from your total testosterone and your SHBG.[1]
Your total testosterone does not tell the whole story. According to the research, free and loosely bound hormone levels usually give a better picture of your situation than the total amount.[1]
The research found large differences between measuring methods. The calculation from total testosterone and SHBG almost matched the direct measurement. A direct immunoassay of free testosterone gave only a fraction of that value. So how your value was measured matters.[1]
| Situation | What it can mean for the measurement | Who it is for |
|---|---|---|
| Low SHBG binding capacity[1] | The calculated value almost matched the direct measurement | Including women with excess hair growth |
| Extremely high SHBG binding capacity[1] | The free androgen index and the direct immunoassay are then not a reliable measure | Including people with a thyroid that works too fast |
| Pregnant[1] | The calculated value comes out lower than the direct measurement | People who are pregnant |
Source: Vermeulen, Verdonck and Kaufman. A comparison of methods for measuring free and bioavailable testosterone, without cut-off values.[1]
The research does not give a limit above which the value is too high. It does show that the measuring method can strongly colour the result, especially if your SHBG is very low or very high. That is why a doctor always looks at how your value was measured.[1]
BioProfile is working on its own zones for each value, from risk to optimal. A doctor reviews those zones before they appear in your result.
SHBG determines how much testosterone is tightly bound. The research looked at blood with an SHBG capacity ranging from low, as in women with excess hair growth, to extremely high.[1]
With a thyroid that works too fast, the research found an extremely high SHBG capacity. That can change the picture of your bioavailable testosterone.[1]
If you are pregnant, oestradiol takes up a large share of the binding sites on SHBG. The immunoassay then overestimates how much SHBG can bind. As a result, the calculated value comes out lower than the direct measurement.[1]
A direct immunoassay of free testosterone gave only a fraction of the true value, and that fraction depended on your SHBG. The calculation from total testosterone and SHBG did turn out to be reliable.[1]
Bioavailable testosterone is included in BioProfile 121 and 150.[2]
The BioProfile editorial team wrote this page on 6 November 2026 from the sources below, which we read on 6 November 2026. Every figure on this page appears in one of those sources, with that source's number next to it. An expert is still reviewing this page; once that review is done, the expert's name and the date of the review will appear at the top. This page offers an explanation, not a diagnosis. Discuss an abnormal value with your GP.
