Understanding the Androgen Index: A Practical Guide for Men

The “androgen index” most people talk about in blood work is the Free Androgen Index (FAI). It’s a simple calculation that estimates how much active (unbound) testosterone is available in the bloodstream. While it’s not perfect, it can be a useful screening tool in men’s health, especially when interpreted alongside symptoms and other lab values.

At True North Metabolic Kitchener-Waterloo Men's Health Clinic, we often use the androgen index as one part of a broader assessment for low testosterone, erectile dysfunction, fatigue, and metabolic issues for patients from Kitchener-Waterloo, Guelph, Ontario, Cambridge, Ontario, and surrounding areas.

What Is the Free Androgen Index?

Most testosterone in your blood is bound to proteins:

• A large portion is bound tightly to Sex Hormone-Binding Globulin (SHBG).

• Another portion is loosely bound to albumin.

• Only a small fraction is truly free (unbound).

The combination of free and loosely-bound testosterone is considered “bioavailable.” The Free Androgen Index is a way to roughly estimate how much testosterone is not locked up by SHBG.

FAI is particularly useful when SHBG is abnormal (low in obesity/insulin resistance, high in hyperthyroidism, liver disease, or certain medications), because total testosterone alone can be misleading.

The Formula: How to Calculate the Androgen Index

The classic formula for the Free Androgen Index is:

FAI = (Total Testosterone ÷ SHBG) × 100

Important details:

• Total testosterone and SHBG should both be in nmol/L for this formula.

• The result is a unitless number (an index, not a direct hormone level).

For example:

• Total Testosterone = 12 nmol/L

• SHBG = 20 nmol/L

FAI = (12 ÷ 20) × 100 = 0.6 × 100 = 60

An FAI of 60 would generally fall within a “normal” male range, although each lab and guideline may use slightly different reference values. That is why interpretation is always done in the context of symptoms, age, and comorbidities.

Step-by-Step: Using FAI in Real Practice

1. Confirm the patient had a morning blood draw (typically 7–10 a.m.), when testosterone is highest and most stable.

2. Check units on the report:

   • If testosterone is in nmol/L and SHBG in nmol/L, use the formula directly.

   • If testosterone is in ng/dL, it must be converted to nmol/L before using the FAI formula.

3. Calculate FAI using the formula above.

4. Interpret alongside:

   • Symptoms (libido, energy, mood, strength, erectile function).

   • Other labs (LH, FSH, prolactin, estradiol, thyroid, metabolic profile).

   • Physical exam findings (body composition, testicular size, gynecomastia, visceral adiposity).

At True North Metabolic Kitchener-Waterloo TRT Clinic, FAI is often combined with calculated free testosterone and bioavailable testosterone to get a more nuanced picture of androgen status before deciding whether testosterone replacement therapy (TRT) is appropriate.

Strengths and Limitations of the Androgen Index

Strengths:

• Easy to calculate—no special software required.

• Helps adjust for abnormal SHBG, which is common in men with obesity, type 2 diabetes, and metabolic syndrome.

• Useful as a screening tool or for trending over time.

Limitations:

• It is still an estimate, not a direct measurement of free testosterone.

• It assumes a standard relationship between SHBG, albumin, and testosterone that may not hold in every individual.

• It’s less precise than advanced calculations that use binding constants or gold-standard methods like equilibrium dialysis.

This is why many modern men’s health practices use FAI as one tool, not the sole decision-maker, and often pair it with more sophisticated free/bioavailable testosterone calculations.

Androgen Index, Metabolic Health, and Weight Loss

Testosterone is closely linked to metabolic health. Men with low testosterone are more likely to have:

• Increased visceral fat

• Insulin resistance and prediabetes

• Reduced muscle mass and strength

• Lower energy and motivation, which can hinder lifestyle change

Because of this, it is logical to integrate androgen assessment into a broader metabolic and weight-loss strategy. At True North Metabolic Kitchener-Waterloo Weight Loss Clinic, the androgen index can help identify men whose hormone status might be holding them back from optimal fat loss, performance, and long-term health. For many patients from Guelph, Ontario and Cambridge, Ontario, correcting underlying metabolic issues (sleep apnea, insulin resistance, nutrition, exercise) can improve both testosterone levels and overall well-being—sometimes without needing TRT.

When to Seek a Men’s Health Assessment

If you have symptoms such as low libido, fatigue, reduced exercise capacity, loss of muscle mass, or erectile difficulties, it can be worth checking:

• Total testosterone

• SHBG

• Free Androgen Index

• Other relevant labs and risk factors

A targeted evaluation, including the androgen index, helps distinguish between primary low testosterone, secondary causes, and situations where lifestyle and metabolic optimization may be the first priority.

Used properly, the Free Androgen Index is a simple, powerful entry point into understanding male hormone health—and a useful tool in comprehensive care at True North Metabolic Kitchener-Waterloo Men's Health Clinic, True North Metabolic Kitchener-Waterloo TRT Clinic, and True North Metabolic Kitchener-Waterloo Weight Loss Clinic.