Free Androgen Index (FAI) Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is the Free Androgen Index?

The free androgen index (FAI) is a calculated ratio that estimates the amount of biologically active (free) testosterone circulating in your blood. It is derived from the formula: FAI = (total testosterone ÷ SHBG) × 100. The result provides a clinically useful proxy for free testosterone without requiring the technically challenging direct free testosterone assay.

Most testosterone in the blood (approximately 98%) is bound to proteins - primarily sex hormone-binding globulin (SHBG) and albumin. Only the unbound (free) fraction can enter cells and exert androgenic effects. Changes in SHBG therefore dramatically affect how much testosterone is biologically available, making FAI a more informative marker than total testosterone alone.

FAI is particularly valuable in women, where it is the preferred first-line test for investigating suspected androgen excess in conditions like PCOS. In men, free testosterone calculated by the Vermeulen equation is generally preferred, but FAI remains useful for screening.

Why Is FAI Tested?

• PCOS diagnosis - elevated FAI is one of the biochemical hallmarks of polycystic ovary syndrome
• Hirsutism and acne - high FAI confirms androgen excess as the underlying cause
• Female hair loss - androgenetic alopecia in women often correlates with elevated FAI
• Menstrual irregularities - FAI helps distinguish hormonal causes of oligomenorrhoea
• Low libido - in both sexes, FAI can reveal whether androgen levels are suboptimal
• Monitoring anti-androgen therapy - FAI tracks treatment response in conditions like PCOS

Normal Ranges

In women, FAI above 5 is generally considered elevated and suggestive of androgen excess. In men, low FAI (<15) may indicate hypogonadism.

What Do High FAI Levels Mean?

• PCOS - the most common cause of elevated FAI in women of reproductive age
• Low SHBG - obesity, insulin resistance, and hypothyroidism lower SHBG, raising FAI even with normal total testosterone
• Congenital adrenal hyperplasia - excess adrenal androgen production
• Androgen-secreting tumours, rare ovarian or adrenal tumours
• Cushing's syndrome, excess cortisol can increase androgen production
• Exogenous androgens - testosterone therapy or anabolic steroid use

What Do Low FAI Levels Mean?

• High SHBG - oestrogen therapy, oral contraceptives, pregnancy, hyperthyroidism, and liver disease all raise SHBG, lowering FAI
• Hypogonadism in men, primary or secondary testosterone deficiency
• Ageing, SHBG rises with age while testosterone declines, reducing FAI
• Anorexia nervosa - malnutrition suppresses gonadal function
• Hyperprolactinaemia - elevated prolactin suppresses the HPG axis

How to Improve Your FAI Levels

• If FAI is too high (women) - address insulin resistance through weight management, exercise, and possibly metformin; combined oral contraceptives lower FAI by raising SHBG
• Reduce refined carbohydrates - insulin resistance is the primary driver of low SHBG and elevated FAI in PCOS
• Regular exercise - both aerobic and resistance training improve insulin sensitivity and SHBG production
• If FAI is too low (men) — investigate underlying causes of low testosterone; optimise sleep, stress, and body composition
• Maintain healthy thyroid function — thyroid disorders significantly affect SHBG and FAI

When Should You Get Tested?

• Women with excess facial or body hair, acne, or thinning scalp hair
• Irregular or absent periods
• Suspected PCOS
• Men with low libido, fatigue, or erectile dysfunction
• As part of a comprehensive hormone panel

Which Lola Health Tests Include FAI?

FAI is calculated from total testosterone and SHBG, both included in our Hormone 7 panel. Available as an add-on with any Lola Health blood test. For best results, women should test on day 2–5 of their menstrual cycle.