Free Androgen Index (FAI) Calculator
Calculate Free Androgen Index (FAI) for PCOS screening. Converts Testosterone and SHBG units for accurate hormonal analysis in women's health.
About
The Free Androgen Index (FAI) estimates the ratio of physiologically active testosterone in the bloodstream. While total testosterone measures the aggregate hormone level, a significant portion binds to Sex Hormone Binding Globulin (SHBG), rendering it biologically inactive. Assessment of SHBG levels helps clinicians differentiate between total androgen load and the actual free fraction available to tissues.
This metric serves as a primary screening tool for hyperandrogenism in women, particularly during the diagnosis of Polycystic Ovary Syndrome (PCOS). An elevated FAI often correlates with symptoms such as hirsutism, acne, and irregular ovulation. In men, the index assists in diagnosing hypogonadism, though calculated free testosterone via equilibrium dialysis remains the gold standard for male subjects. Results require distinct interpretation based on biological sex due to the vast baseline differences in androgen production.
Formulas
The FAI is a unitless ratio derived from Total Testosterone and SHBG. To maintain accuracy, both variables must be converted to molar units (nmol/L) before calculation.
Where:
- Ttotal is Total Testosterone in nmol/L.
- SHBG is Sex Hormone Binding Globulin in nmol/L.
- If T is measured in ng/dL, the conversion factor is 0.0347.
Reference Data
| Subject Category | FAI Reference Range | Clinical Interpretation |
|---|---|---|
| Adult Female (Premenopausal) | 0.5 - 6.5 | Normal physiological range. |
| Adult Female (PCOS Risk) | > 7.0 | Possible hyperandrogenism; PCOS investigation warranted. |
| Adult Female (Postmenopausal) | 0.4 - 4.5 | Expected decline in ovarian androgen production. |
| Adult Male (Healthy) | 30 - 150 | Normal androgen status. |
| Adult Male (Hypogonadism) | < 30 | Indicates low bioavailable testosterone. |