Free Androgen Index

The normal free androgen index (FAI) is generally considered below about 4–5% (FAI = total testosterone [nmol/L] ÷ SHBG [nmol/L] × 100). Values above approximately 4–5% suggest biochemical hyperandrogenism, often seen in PCOS. Reference ranges and cutoffs vary by lab and assay, so interpret results with clinical findings and local laboratory standards.

A high androgen index means your body has relatively elevated male hormones (androgens) compared with estrogens. It often signals conditions such as polycystic ovary syndrome, adrenal or ovarian androgen-secreting tumors, or congenital adrenal hyperplasia. Symptoms include acne, excess facial/body hair, irregular periods, hair thinning, and fertility or metabolic issues. Doctors will evaluate with blood tests, imaging and targeted treatment to address the cause.

The free androgen index (FAI) is calculated from a blood (serum) sample measuring total testosterone and sex hormone–binding globulin (SHBG). A venous blood draw is usually done in the morning because testosterone levels vary through the day. The FAI estimates the proportion of biologically active (free) testosterone and helps assess androgen excess conditions such as polycystic ovary syndrome, hirsutism, or unexplained virilization.

Calculate FAI by dividing total testosterone (in nmol/L) by SHBG (in nmol/L) and multiplying by 100: FAI = (total testosterone ÷ SHBG) × 100. If testosterone is reported in ng/dL, convert to nmol/L by multiplying by 0.0347 before calculating. FAI is expressed as a percentage; interpretive ranges vary, so review results with a clinician.

Polycystic ovary syndrome is usually characterized by elevated androgens (hyperandrogenism), which cause symptoms like hirsutism, acne, and hair thinning. Some women may have normal total testosterone but increased free or bioavailable androgens. Insulin resistance commonly associated with PCOS stimulates ovarian androgen production, so diagnosis uses clinical signs plus biochemical testing.

The Free Androgen Index (FAI) can be a useful adjunct to detect biochemical hyperandrogenism in suspected PCOS—especially when SHBG is low or direct testosterone assays are limited. However, FAI alone cannot confirm PCOS. Diagnosis requires clinical signs (oligo/anovulation, hirsutism), ultrasound findings and exclusion of other causes. Interpret FAI alongside symptoms and specific hormone testing.