Free Androgen Index (FAI) Calculator
Free Androgen Index (FAI) Calculator
Calculate the free androgen index from total testosterone and SHBG, a simple but crude surrogate used mainly in the assessment of women.
Free Androgen Index (FAI)
Simple ratio, interpret with cautionTotal testosterone 3.2 nmol/L, SHBG 28 nmol/L
Formula
- Total testosterone
- nmol/L — must be in the same unit family as SHBG
- SHBG
- nmol/L
- result
- a dimensionless index, not a concentration
Worked example
Total testosterone 3.2 nmol/L, SHBG 28 nmol/L
3.2 ÷ 28 = 0.1143
0.1143 × 100 = 11.4 FAI
FAI compared with calculated free testosterone
| FAI | Vermeulen free testosterone | |
|---|---|---|
| Inputs | Total T, SHBG | Total T, SHBG, albumin |
| Accounts for albumin binding | No | Yes |
| Performs well in men | Poorly | Well |
| Performs well at high SHBG | Poorly | Well |
| Typical use | Screening women for hyperandrogenism | Any setting where albumin is available |
What the FAI is good for, and what it is not
The free androgen index is a crude surrogate for free testosterone, calculated as the ratio of total testosterone to SHBG. It is used mainly in women, where a value above about 5 supports biochemical hyperandrogenism in the assessment of polycystic ovary syndrome alongside the clinical picture and ultrasound findings.
It should be used with its limitations stated plainly. It performs poorly in men and at high SHBG, because it does not model the albumin-bound fraction the way the Vermeulen equation does — the two markers can disagree, and where albumin is available the Vermeulen calculation is the more reliable test in both sexes. FAI is best treated as a screening tool rather than a definitive one.
Both values entered must be in nmol/L. Mixing a testosterone result reported in ng/dL with an SHBG in nmol/L is a common and large error, and produces an index that is wrong by roughly a factor of thirty rather than a small rounding difference. Check the units on the report before entering them.
The combined oral contraceptive roughly doubles SHBG through its oestrogen component, which in turn roughly halves the FAI. Testing should be done off the combined oral contraceptive for at least three months wherever possible, or the result interpreted with that suppression explicitly in mind.
Frequently asked questions
What is a normal free androgen index?
Below about 5 in women is generally regarded as normal. Between 5 and 10 supports biochemical hyperandrogenism in the right clinical context, and values above 10 are more strongly suggestive.
Can the FAI be used in men?
It performs poorly in men because it ignores albumin binding. The Vermeulen calculation, which includes albumin, is the better choice whenever it is available.
Why is my FAI wrong by a huge margin?
The commonest cause is a units mismatch — entering testosterone in ng/dL against SHBG in nmol/L. Both values must be in nmol/L for the formula to be valid.
Does the contraceptive pill affect the FAI?
Yes, substantially. The combined oral contraceptive roughly doubles SHBG, which roughly halves the FAI. Testing off it for at least three months gives a more interpretable result.
Related calculators
References
- Teede HJ et al. Recommendations from the 2023 international evidence-based guideline for the assessment and management of PCOS. Fertil Steril. 2023;120(4):767–93.
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666–72.