Free Androgen Index (FAI)

Your body carries testosterone in the blood in two main forms: bound (mostly attached to SHBG and albumin) and free (unbound). Only a small portion is free or loosely bound — and that’s the part that can enter cells and do the work (supporting muscle, bone, libido, motivation, and energy). When SHBG is high, it ties up more testosterone, often lowering the amount available; when SHBG is low, a larger share of testosterone remains available.

FAI helps explain symptoms when total testosterone alone doesn’t. Two men can have the same total testosterone, but the one with higher SHBG may have lower FAI and feel the effects of androgen shortfall (fatigue, low libido, reduced strength).

Conversely, very low SHBG can drive high FAI, which may track with acne, oily skin, irritability, or hair thinning in some men. FAI is useful in the context of hypogonadism (low androgen function) and fertility evaluation, and it correlates with aspects of body composition, bone density, and cardiometabolic health.

Dietary factors:

• Refined carbs + added sugars (soft drinks, sweets, frequent pastries) can worsen insulin resistance, which often lowers SHBG and can raise FAI—sometimes with androgen-excess–type symptoms.
  

• Very low calorie or very low protein intake can reduce testosterone production, lowering FAI.
  

• Mediterranean-style eating (extra-virgin olive oil, legumes, whole grains, nuts, plenty of veg, oily fish 2–3×/week) supports insulin sensitivity and healthier SHBG/testosterone balance.
  

• Alcohol (especially daily or >7 drinks/week) can impair liver handling of hormones, shift SHBG, and blunt testosterone production.
  

Lifestyle factors:

• Resistance training (2–4 sessions/week) and regular aerobic work improve insulin sensitivity and body composition — tending to normalise SHBG and support testosterone → healthier FAI.
  

• Chronic stress and poor sleep lower testosterone signals; 7–9 hours of consistent sleep helps.
  

• Excess adiposity (particularly abdominal fat) lowers SHBG and may raise FAI but with unhealthy metabolic context.
  

Related biomarkers:

• Total testosterone: the numerator in FAI — if low, FAI often falls unless SHBG is also very low.
  

• SHBG: the denominator — high SHBG (e.g., with aging, hyperthyroidism, low insulin) can reduce FAI; low SHBG (e.g., with insulin resistance) can raise FAI.
  

• LH (pituitary signal), prolactin, thyroid markers, HbA1c/fasting insulin: upstream drivers that shape testosterone and SHBG.
  

Micronutrients:

• Zinc (oysters, beef, pumpkin seeds) and magnesium (leafy greens, nuts, legumes) support steroid hormone production and insulin action, indirectly influencing FAI.
  

Environmental influences:

• Endocrine disruptors (e.g., BPA from some plastics/thermal receipts, certain pesticides) can interfere with androgen pathways.

• Liver health (fatty liver, high alcohol, some medications) alters SHBG and hormone metabolism.

• Low FAI can present as low libido, reduced morning erections, low mood, brain fog, decreased muscle/strength, increased fat mass.
  

• High FAI can correlate with acne/oily skin, irritability, hair thinning, or sleep changes (context matters).
  

• Related diagnoses to consider with abnormalities: hypogonadism, thyroid disorders, obesity/insulin resistance, liver conditions, hyperprolactinaemia, medication effects.
  

• See a clinician promptly for persistent sexual dysfunction, infertility concerns, breast changes, major mood shifts, or if considering hormone therapy.

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