Hormones
Free Androgen Index (FAI)
Testosterone to SHBG ratio
Included in Baseline and Complete.
Compare membershipsYour testosterone and SHBG, combined into one estimate.
The Free Androgen Index is calculated from your total testosterone and SHBG results. It estimates how much testosterone is free to act on your tissues and is used most often when assessing androgen levels in women.
A simple ratio with a specific job.
Because SHBG holds testosterone in reserve, comparing total testosterone with SHBG gives a rough sense of how much is available. The 2023 international PCOS guideline lists the Free Androgen Index as one way to estimate free testosterone when assessing polycystic ovary syndrome.
In men, the index is less reliable, so clinicians more often use free testosterone, either calculated from total testosterone, SHBG, and albumin or measured directly. The index carries the same timing effects as testosterone, so a morning draw gives the most comparable results.
Testosterone and SHBG together give a rough measure of what's available.
Questions
What does a higher Free Androgen Index mean for women?
A higher index may suggest more active androgen, which can be associated with acne, excess hair growth, or irregular periods. It's one input when PCOS is being considered, alongside your symptoms, cycle history, and other results such as total testosterone and DHEAS.
Why might my index change if my testosterone didn't?
The index depends on SHBG too. SHBG can shift with body weight, insulin resistance, thyroid function, and estrogen-containing medicines, which moves the index even when total testosterone stays the same.