Free androgen index: the ratio behind the raw numbers
The free androgen index, or FAI, isn't a separate blood draw — it's a calculation. It combines your total testosterone with your SHBG to estimate how much testosterone is likely free and active. Clinicians often look at it alongside the raw figures because it captures something neither number quite shows on its own.
What this test measures
The free androgen index is a ratio worked out from two measured values: total testosterone and SHBG. Because SHBG binds testosterone, the same total can mean very different amounts of free, active hormone depending on how much carrier protein is present. The FAI is an attempt to express that in a single figure. It's a calculated estimate, not a direct measurement, and the way it's expressed and interpreted varies by lab and assay — so there's no universal threshold to quote, and we don't.
Why it comes up in PCOS care
Higher androgen activity is part of the PCOS picture, and the FAI is one way clinicians try to capture it more faithfully than the total testosterone alone. Because it depends on both inputs, it can flag raised androgen activity that a normal-looking total might hide. The NHS lists hormone blood tests among the steps in diagnosing PCOS, and the FAI is read as one part of that — alongside symptoms like acne and excess hair growth, never as a standalone answer.
Keeping results across years
Because the FAI is built from two values, it's only as useful as the records behind it — and those records are exactly what gets lost when you switch GP or your clinic changes systems. A dated vault that holds your testosterone, SHBG and the resulting index together, entry by entry, lets you and a clinician see whether your androgen picture is steady or shifting. One ratio in isolation says little; the same ratio tracked over years is a trend worth discussing.
PCOS Diary keeps testosterone, SHBG and the resulting index together in a dated lab vault, so the whole androgen picture reads as one trend in your PDF.
Common questions
- Is the free androgen index a blood test?
- Not directly — it's calculated from your total testosterone and SHBG, both of which are measured from blood. It estimates how much testosterone is likely free and active, which the total alone can't show.
- What's a normal free androgen index?
- There's no single figure to quote. How the index is expressed and interpreted varies by lab and assay, and it's read alongside your other results and symptoms by a clinician rather than against a fixed cut-off.