Summary
The Free Androgen Index (FAI) is a calculated ratio that estimates the amount of biologically available testosterone, derived from total testosterone and SHBG. It is particularly useful in women for assessing androgen excess in conditions like PCOS, where total testosterone alone can be misleading. FAI rises when SHBG is low, even if total testosterone is normal.
FAI is calculated as: (total testosterone ÷ SHBG) × 100. Because SHBG determines how much testosterone is bound and inactive, this ratio estimates the proportion of testosterone that is free and active.
In women, FAI is a practical and widely used marker of androgen excess. Many women with PCOS have a normal total testosterone but a low SHBG (driven by insulin resistance), resulting in a raised FAI and androgenic symptoms such as acne, hirsutism, and scalp hair thinning. FAI captures this where total testosterone alone would not.
FAI is a useful screening estimate rather than a precise measure of free testosterone — calculated free testosterone (which also includes albumin) is more accurate, but FAI is simple and clinically valuable, especially in women.
What It Is
The Free Androgen Index is a dimensionless ratio: FAI = (total testosterone [nmol/L] ÷ SHBG [nmol/L]) × 100. It provides a quick estimate of bioavailable androgen by accounting for the proportion of testosterone bound to SHBG.
FAI performs best in women, where it correlates reasonably with free testosterone and is widely used in PCOS assessment. In men, where testosterone and SHBG are higher, FAI is less reliable, and calculated free testosterone is preferred.
Reference ranges (women): typically < 5 is normal, with values above this suggesting androgen excess (some laboratories use slightly different cut-offs). Because it depends on both inputs, FAI rises with either increased testosterone or decreased SHBG.
Functions
Androgen excess screening
Estimates bioavailable androgen to detect androgen excess in women, even when total testosterone appears normal.
PCOS assessment
A raised FAI, driven by low SHBG, is a common and useful finding supporting a diagnosis of PCOS.
Capturing SHBG effects
By incorporating SHBG, FAI reveals androgen excess from low SHBG that total testosterone alone would miss.
Simple bioavailable androgen index
Provides a quick, practical estimate of active androgen from two routine measurements.
Reference Ranges
Free Androgen Index (FAI)
Measured in ratio (×100)| Status | Range (ratio (×100)) | What it means |
|---|---|---|
| Normal (women) | < 5 | Normal bioavailable androgen in women — androgen excess unlikely. |
| Borderline | 5–7 | Borderline — possible mild androgen excess; interpret with symptoms and other markers. |
| Elevated | > 7 | Elevated — suggests androgen excess, commonly seen in PCOS. |
FAI cut-offs vary by laboratory and assay. FAI is most reliable in women; in men, calculated free testosterone is preferred. Always interpret alongside total testosterone, SHBG, and clinical symptoms.
Symptoms of Imbalance
FAI reflects bioavailable androgen; symptoms relate to androgen excess (high FAI) or deficiency (low FAI).
- Low libido
- Fatigue
- Reduced wellbeing
- Often no specific symptoms in women
- Acne and oily skin
- Hirsutism (excess facial and body hair)
- Scalp hair thinning (female-pattern)
- Irregular periods
- Difficulty conceiving (PCOS)
Causes of Imbalance
- High SHBG (ageing, hyperthyroidism, oestrogen, liver disease)
- Low total testosterone
- Hypogonadism
- PCOS (low SHBG with normal or high testosterone)
- Insulin resistance and obesity (lowering SHBG)
- Congenital adrenal hyperplasia
- Androgen-secreting tumours (rare)
- Anabolic steroid use
FAQs
FAI is calculated as total testosterone divided by SHBG, multiplied by 100: FAI = (total testosterone ÷ SHBG) × 100, using values in nmol/L. Because it depends on both inputs, FAI rises when testosterone increases or — importantly — when SHBG falls. This is why both total testosterone and SHBG must be measured to obtain it.
Many women with PCOS have a normal total testosterone but a low SHBG, often driven by insulin resistance. A low SHBG means more testosterone is free and active, producing androgenic symptoms like acne and excess hair growth. FAI captures this by combining both values, so it can reveal androgen excess that the total testosterone alone would miss — making it a valuable marker in PCOS assessment.
FAI is a simpler estimate and works well in women, but calculated free testosterone — which also accounts for albumin using the Vermeulen equation — is more precise. In men, where testosterone and SHBG are higher, FAI is less reliable and calculated free testosterone is preferred. FAI is best regarded as a practical screening index rather than a definitive measure.
FAI rises either when total testosterone increases or when SHBG decreases. In women with PCOS, the most common reason is a low SHBG caused by insulin resistance and excess weight, even when total testosterone is only modestly raised or normal. This is why a high FAI often accompanies metabolic features, and why addressing insulin resistance can help lower it.
References
- Teede HJ, et al. International evidence-based guideline for the assessment and management of polycystic ovary syndrome. Hum Reprod. 2018;33(9):1602–1618. View source
- Vermeulen A, et al. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666–3672. View source
- Martin KA, et al. Evaluation and treatment of hirsutism in premenopausal women: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(4):1233–1257. View source