The Androgen Index, also called the Free Androgen Index (FAI), is a calculated value used to estimate the amount of active (free) testosterone in the blood.
It is mainly used to evaluate androgen excess.
🔬 How It Is Calculated
Most testosterone in blood is bound to SHBG (Sex Hormone-Binding Globulin).
Only a small portion is free and biologically active.
📌 Formula:
FAI=Total TestosteroneSHBG×100FAI = \frac{\text{Total Testosterone}}{\text{SHBG}} \times 100
(Units must match, usually nmol/L.)
🩺 Why Doctors Order It
Commonly used to evaluate:
Excess hair growth (hirsutism)
Acne
Irregular periods
Suspected Polycystic ovary syndrome (PCOS)
Androgen disorders
📊 Interpretation (General Guide)
| FAI Result | Meaning |
|---|---|
| Normal | Normal androgen activity |
| High | Androgen excess |
| Low | Possible androgen deficiency (more common in males if low testosterone) |
Normal ranges vary by lab and sex.
📝 Key Points
It is a calculated value, not a direct measurement.
Helps estimate free (active) testosterone.
More useful when SHBG levels are abnormal.
If you want, share your total testosterone and SHBG values, and I can calculate and explain your androgen index.
