Free androgen index (FAI)

Women: below 5 ratio · Men: 30 to 150 ratio

It is mainly used in women, where most UK labs print an upper limit of about 5 (some 7 or 8), and in men a range of roughly 30 to 150 is typical, though few labs print a male range.

The free androgen index is a calculation, not a separate measurement: the total testosterone divided by the SHBG, multiplied by 100. It gives a rough idea of how much testosterone is free to act on the body. It is most used in women, where measuring free testosterone directly is unreliable.

Doctors request it in women with excess hair growth, acne, hair thinning or irregular periods, most often when polycystic ovary syndrome is being looked into. It is less useful in men, where calculated free testosterone is preferred.

• Polycystic ovary syndrome, the commonest reason in women
• Low SHBG from heavier body weight, insulin resistance or an underactive thyroid
• Testosterone or anabolic steroid use
• Rarely, ovarian or adrenal conditions that make extra testosterone

• High SHBG, for example on the combined pill, on HRT, in an overactive thyroid or liver disease
• Older age in men
• Testicular, pituitary or hypothalamus problems in men
• Low body weight

• It moves whenever SHBG moves, so the combined pill lowers it and weight gain raises it without any change in testosterone
• It carries the time-of-day and repeatability variation of the testosterone it is calculated from
• Labs use different cut-offs, commonly 5, 7 or 8 in women

It comes from the same sample as testosterone and SHBG, ideally a morning one. Timing in the cycle matters little.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.