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Free Androgen Index (FAI) – Meaning & Normal Values

The Free Androgen Index (FAI) is a calculated lab value that reflects the level of biologically active androgens in the blood, helping to identify hormonal disorders.

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Things worth knowing about "Free Androgen Index"

The Free Androgen Index (FAI) is a calculated lab value that reflects the level of biologically active androgens in the blood, helping to identify hormonal disorders.

What Is the Free Androgen Index?

The Free Androgen Index (FAI) is a calculated laboratory parameter used to estimate the amount of biologically active testosterone circulating in the blood. Testosterone – the primary male sex hormone (androgen) – travels through the bloodstream mostly bound to transport proteins, especially Sex Hormone-Binding Globulin (SHBG). Only a small fraction remains unbound (free) and is biologically active.

Because direct measurement of free testosterone is technically complex and prone to inaccuracies, the FAI serves as an indirect estimate. It is calculated using total testosterone and SHBG levels as follows:

FAI (%) = (Total Testosterone ÷ SHBG) × 100

The FAI is particularly relevant in gynecological and endocrinological diagnostics, especially for the evaluation of hormonal imbalances in women.

When Is the FAI Measured?

The Free Androgen Index is commonly used in the workup of the following conditions:

Hirsutism: excessive body or facial hair growth in women following a male pattern
Acne and oily skin: potential signs of elevated androgen activity
Polycystic Ovary Syndrome (PCOS): the most common hormonal disorder in women of reproductive age
Menstrual irregularities and amenorrhea (absence of menstruation)
Female infertility
Androgen deficiency in men: for example, in cases of hypogonadism
Monitoring during hormone therapy

Reference Values and Interpretation

Reference ranges for the FAI vary by sex and laboratory. The following values serve as general orientation:

Women: an FAI of approximately 0.5 to 5% is considered normal. Values above 5% may indicate hyperandrogenemia (elevated androgen activity).
Men: FAI values are physiologically higher, typically ranging from 40 to 90%. Very low values may suggest androgen deficiency.

An elevated FAI in women is an important diagnostic criterion for Polycystic Ovary Syndrome (PCOS) and other conditions associated with excessive androgen production. A low FAI in men may point to hypogonadism, while a low FAI in women may reflect reduced androgen activity.

Factors That Influence the FAI

Several factors can affect the FAI, as both total testosterone and SHBG levels depend on various variables:

Factors that increase SHBG (lower the FAI): estrogens, oral contraceptives, thyroid hormones, liver disease
Factors that decrease SHBG (raise the FAI): insulin resistance, obesity, hypothyroidism, corticosteroids
Time of day: testosterone follows a circadian rhythm; blood samples should ideally be taken in the morning
Menstrual cycle phase: androgen levels fluctuate throughout the menstrual cycle in women

Diagnostic Value and Limitations

The FAI is a practical tool in hormonal diagnostics but has certain limitations. It is an estimate and does not replace the direct measurement of free testosterone by equilibrium dialysis, which is considered the gold standard. However, the FAI is widely used in clinical research and practice due to its ease of calculation and strong clinical correlation.

Interpretation of the FAI should always be made in the context of clinical findings, additional hormones (e.g., LH, FSH, DHEA-S), and the individual symptom profile. An elevated or reduced value alone is not sufficient for a definitive diagnosis.

References

Vermeulen A., Verdonck L., Kaufman J.M. - A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology & Metabolism, 84(10):3666-3672 (1999). PubMed PMID: 10523012.
Rotterdam ESHRE/ASRM-Sponsored PCOS Consensus Workshop Group - Revised 2003 consensus on diagnostic criteria and long-term health risks related to polycystic ovary syndrome. Human Reproduction, 19(1):41-47 (2004). PubMed PMID: 14688154.
Goodman N.F. et al. - American Association of Clinical Endocrinologists, American College of Endocrinology, and Androgen Excess and PCOS Society Disease State Clinical Review: Guide to the Best Practices in the Evaluation and Treatment of Polycystic Ovary Syndrome. Endocrine Practice, 21(12):1415-1426 (2015).

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