Free Androgen Index (FAI)
Hormones marker
Free Androgen Index
Free Androgen Index (FAI)
Ratio of total testosterone to SHBG, expressed as a percentage: (Total T / SHBG) × 100. Estimates the proportion of bioavailable testosterone. More useful than total T alone because SHBG status dramatically affects androgen exposure.
PED Notes
Auto-calculated when both Testosterone and SHBG are present in a blood test. Normal male range is 30-150%. On AAS/TRT, FAI is typically very high (>200%) — this is expected and not actionable. The main clinical utility is off-cycle or on TRT: a low FAI (<30%) despite normal total T points to high SHBG as the cause of hypogonadal symptoms (low libido, erectile dysfunction, fatigue, poor recovery). On oral AAS that crush SHBG (Anavar, Winstrol, Proviron), FAI can be extremely high (>500%) even with moderate total T — this means high free androgen exposure and explains androgenic side effects despite 'normal' total T levels.
This index is auto-computed — it is not directly manageable.
Adjust the individual components (Testosterone and/or SHBG) instead.
If FAI is low (<30%) — high SHBG binding too much testosterone:
- This is the most common cause of sexual dysfunction (ED, low libido) despite "normal" total T
- Boron — 10mg/day (lowers SHBG ~10% within 1 week; most evidence-supported)
- Stinging Nettle Root — 300-600mg/day (may reduce SHBG binding)
- Address caloric deficit — chronic dieting raises SHBG
- Ensure adequate carbohydrate intake — very low-carb diets elevate SHBG
- Check thyroid function — hyperthyroidism significantly raises SHBG
- If persistent: consider low-dose TRT or SERM therapy (enclomiphene 12.5mg/day)
If FAI is very high (>200%) off-cycle — SHBG is suppressed:
- Common with oral AAS use, insulin resistance, obesity, or hypothyroidism
- High free androgen exposure increases risk of: acne, hair loss, prostate stimulation
- Discontinuing oral AAS typically restores SHBG in 4-8 weeks
- Address insulin resistance: reduce refined carbohydrates, increase fibre, exercise
Sexual dysfunction context:
- Low FAI is strongly associated with erectile dysfunction, reduced libido, and poor sexual satisfaction — even when total T is within range
- If experiencing ED/low libido with normal total T, check FAI — if <30%, SHBG management is the intervention, not testosterone replacement
- High SHBG (low FAI) is more common in: older men, lean dieters, hyperthyroid states, and heavy alcohol users
- On TRT: if libido/erections are poor despite good total T levels, very high SHBG may be the issue — consider more frequent injections (EOD) or adding a small dose of oral androgen to lower SHBG
References:
- Naghii, M. R., Mofid, M., Asgari, A. R., Hedayati, M., & Daneshpour, M.-S. (2011). Comparative effects of daily and weekly boron supplementation on plasma steroid hormones. Journal of Trace Elements in Medicine and Biology, 25(1), 54-58. DOI: 10.1016/j.jtemb.2010.10.001
- Huijben, M., Lock, M. T. W. T., de Kemp, V. F., de Kort, L. M. O., & van Breda, H. M. K. (2022). Clomiphene citrate for men with hypogonadism: A systematic review and meta-analysis. Andrology, 10(3), 451-469. DOI: 10.1111/andr.13146
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