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The free androgen index is a calculation based on your testosterone and your SHBG, the protein that binds testosterone in your blood. A high or low index can point to more or less testosterone relative to SHBG, but the index also depends on how much SHBG you have.[1]
On this page, you can read what the free androgen index is, what the guidelines say about a value that is too low or too high, what affects your value and which values are best read alongside it.
Also known as: FAI, free androgen index. At the lab: not applicable.
Part of the testosterone in your blood is bound to SHBG. The part that is free or only loosely bound is the easiest for your body to use.[1]
The free androgen index tries to estimate that free portion with a simple sum: 100 times your testosterone, divided by your SHBG. For this, SHBG is measured with an immunoassay.[1]
The index is one of several ways to estimate available testosterone. According to the researchers, it had not yet been fully tested when it was already in use.[1]
According to the researchers, the amount of free and loosely bound hormone usually reflects how someone is doing better than the total amount in the blood. That is why a good estimate of that portion matters.[1]
The most precise methods take a lot of time and fit poorly into routine lab work. That is why quick estimates such as the index are appealing.[1]
In the study, the index did relate to measured free testosterone, but the relationship between the two changed with the SHBG level. That is why a doctor always reads the index together with your testosterone and your SHBG.[1]
| Situation according to the source | What it can indicate for the index | Who it is for |
|---|---|---|
| Low SHBG[1] | The index relates to free testosterone differently than it does at average SHBG | For example, women with excessive hair growth |
| Very high SHBG[1] | The index relates to free testosterone differently than it does at average SHBG | For example, people whose thyroid is overactive |
| Pregnant[1] | Estradiol occupies a large share of SHBG, so calculated free testosterone comes out lower than measured | People who are pregnant |
Source: Vermeulen, Verdonck and Kaufman, a study of simple estimates of free testosterone in serum. The source gives no cut-off values for the index.[1]
A high index can point to a lot of testosterone relative to SHBG. Because the index shifts with SHBG, a low SHBG can also push the index up. The researchers saw low SHBG, for example, in women with excessive hair growth.[1]
A low index can point to little testosterone relative to SHBG, or to a high SHBG. The researchers saw very high SHBG, for example, in people whose thyroid is overactive.[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 sits below the line in the sum. The lower your SHBG, the higher the index comes out, and the other way round. The researchers saw that, because of this, the index comes out differently from measured free testosterone at low and high SHBG.[1]
With an overactive thyroid, the researchers found very high SHBG. That can pull the index down.[1]
In people who are pregnant, estradiol occupies a large share of the binding sites on SHBG. The immunoassay then overestimates how much SHBG can bind testosterone, which makes calculations based on SHBG less accurate.[1]
Free androgen index 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.
