What is FAI?
Written by Dr Robin Wang
FAI stands for Free Androgen Index. Androgen is another word for male sex hormone. It is not a measurement but a calculation: Total Testosterone divided by SHBG. Most testosterone travels stuck to a carrier protein and only the part travelling free can enter tissues, and FAI is a quick estimate of how much of it is free.
Why does FAI matter?
Most testosterone in the blood is bound to SHBG, the protein that carries sex hormones around, and while it is bound it cannot enter tissues. The total therefore overstates how much is free to act, and the index corrects for that.
How does FAI relate to other markers?
Total Testosterone is the numerator and SHBG the denominator, and nothing else enters the calculation. Free Testosterone estimates the same unbound share but includes albumin, which binds testosterone loosely as well, so it is the closer estimate.
What affects FAI?
Anything that changes testosterone production moves the index, including LH, the pituitary signal that drives the testes and ovaries to make it. So does anything that changes SHBG: thyroid activity, liver function, body weight and oestrogen exposure.
Why Do Health tests FAI once a year
Total Testosterone rises and falls within a day around its morning peak, and SHBG moves over months. FAI therefore carries testosterone's daily swing and SHBG's slow drift in the same figure. Yearly spacing follows SHBG, the slower of the two, so a difference still present a year later points to SHBG and not to the hour the blood was taken. This is Do Health's monitoring schedule, not a general recommendation for everyone.
How does my FAI compare?
Source
Society for Endocrinology Clinical Practice Guideline for the Evaluation of Androgen Excess in Women
pmc.ncbi.nlm.nih.govReviewed
Dr Robin Wang · 01/10/2026
British physician, Clinical Lead at Do Health
More in Hormones
All the testosterone in the blood, showing how much the body makes.
Learn more about Total TestosteroneThe part of testosterone the body can use.
Learn more about Free TestosteroneA hormone from the brain that controls milk production and can affect periods and fertility when high.
Learn more about ProlactinA hormone from the brain that tells the ovaries to develop eggs and the testes to make sperm.
Learn more about FSHA hormone from the brain that tells the ovaries to release an egg and the testes to make testosterone.
Learn more about LHThe main form of oestrogen, which controls the menstrual cycle and helps protect bones.
Learn more about OestradiolA protein that carries sex hormones in the blood and controls how much is free for the body to use.
Learn more about SHBGOne membership.
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