Ever looked at your hormone results and thought, “Wait, what does any of this actually mean?” You’re not alone. And if terms like “free testosterone” and “SHBG” make your eyes glaze over, the Free Androgen Index (FAI) is about to earn its keep.
FAI is a simple calculation with a powerful purpose: it estimates how much active androgen signal your body is seeing. That matters in real life. Think irregular periods or chin hair you didn’t order. Think thinning scalp hair, acne that won’t quit, or low libido and lagging recovery. By the end of this guide, you’ll know what FAI measures, why it shifts, how to read highs and lows, and where it fits with other biomarkers. Ready to make the numbers talk?
What Free Androgen Index (FAI) Actually Measures
FAI estimates the “free,” or biologically available, portion of testosterone by comparing total testosterone to a binding protein called SHBG. In plain terms, it tells you how much of the gas pedal is actually reachable by your cells, not just sitting in the tank.
Under the hood: FAI = (Total testosterone ÷ SHBG) × 100, typically with both measured in nmol/L. Testosterone comes from the ovaries and adrenals in women, and primarily the testes in men. SHBG, made in the liver, acts like a sponge that soaks up testosterone. When SHBG is high, less testosterone is free. When SHBG is low, more is free. So a rising FAI usually means more androgen signal or less SHBG. A falling FAI suggests less androgen signal or more SHBG. It’s an estimate, not a direct measurement, and context is everything.
The Science, Made Simple
Picture SHBG as the bouncer at a crowded club. Total testosterone is the whole line outside. Only the unescorted guests get in. FAI approximates how many make it past the rope and onto the dance floor where receptors live.
What changes the bouncer’s mood? Insulin can push SHBG down, which lets more testosterone stay free. Estrogen and thyroid hormone lift SHBG, which tightens access. The liver sets SHBG policy, so anything that changes liver metabolism can tip the balance.
Stress, sleep, and training load also play roles. Testosterone follows a circadian rhythm, higher in the morning. Sleep debt can mute that rise. Intense workouts can cause short-term bumps. In women, ovarian and adrenal output shifts with the menstrual cycle, and hormonal contraception can raise SHBG. Even aging changes the choreography. The punchline: single snapshots can mislead. Patterns across time, anchored to symptoms and life events, tell the real story.
Normal Versus Optimal
“Normal” lab ranges are reference intervals built from large populations. They show where most people land, not whether you’re thriving. For FAI, there’s no universal “optimal” cut-off that fits everyone. Different labs use different assays and units, and the formula hinges on both total testosterone and SHBG accuracy.
Interpretation depends on sex, age, and life stage. In women, FAI is often used to screen for hyperandrogenism, especially in suspected polycystic ovary syndrome (PCOS). In men, FAI adds less value because total and free testosterone directly guide evaluation. Hormonal contraception, pregnancy, thyroid status, liver health, and metabolic status can all shift FAI without reflecting a primary androgen problem. Treat your result as a conversation starter, not a verdict.
Interpreting High and Low Levels
When Levels Run High
High FAI means the balance tilts toward more available androgen signal or lower SHBG. In women, that often points to hyperandrogenism. The common story is PCOS, where insulin resistance can suppress SHBG and ovarian androgen output runs higher. The picture can include irregular periods, acne, or unwanted hair growth. Other possibilities include nonclassic congenital adrenal hyperplasia, rare androgen-secreting tumors, or medication effects.
But here’s the twist: FAI can climb even when total testosterone is average if SHBG is low. SHBG commonly drops with insulin resistance, higher visceral fat, hypothyroidism, and certain medications. It can also run low in some liver and kidney conditions. So a high FAI is a signal, not a sentence. Pair it with total testosterone measured by a high-quality method (LC-MS/MS is the clinical gold standard), SHBG, and, when needed, DHEA-S and 17-hydroxyprogesterone to map the source. Persistent elevation across repeat tests, plus symptoms, is what moves clinicians toward diagnoses.
When Levels Run Low
Low FAI reflects less available androgen signal or high SHBG. In women, FAI may fall with estrogen-containing contraception, pregnancy, hyperthyroidism, or significant weight loss. Some women with low androgen tone report low libido or low energy, but the research is mixed, and response to treatment is individualized. In men, a low FAI can appear with high SHBG even when total testosterone is midrange, which is why measuring or calculating free testosterone directly is often preferred.
Again, context is king. SHBG rises with estrogen exposure, hyperthyroidism, and some liver conditions. It can also rise with low insulin and lower body fat. Acute illness, overtraining, severe calorie deficit, and medications like glucocorticoids or opioids can reduce androgen production regardless of SHBG. If a low FAI shows up once, retest in the morning, in a stable health window, and line it up with symptoms before drawing conclusions.
Longevity and Whole-Body Health
FAI isn’t a longevity score, but it tracks with systems that matter for health span. In women, chronically high androgen activity, as seen in many with PCOS, ties to insulin resistance, dyslipidemia, and higher long-term cardiometabolic risk. In men, very low androgen availability correlates with low muscle mass, anemia, and lower bone density. Here, FAI is more a weather vane than a barometer. It reflects changes in SHBG — a protein sensitive to liver health, thyroid status, estrogen exposure, and metabolic signals. Watch the trend line and you’ll often see broader physiology shifting in the background.
How to Improve or Optimize Your Levels
Nutrition
Think levers, not hacks. The liver makes SHBG, and insulin signaling tells the liver how much to make. Diet patterns that improve insulin sensitivity can raise SHBG over time, which lowers FAI when it’s elevated from low SHBG. That usually looks like fiber-rich carbohydrates, adequate protein, and unsaturated fats anchored to energy balance that trims visceral fat. Excess added sugars and chronic surplus calories can drive liver fat and nudge SHBG down. Alcohol can lower SHBG in some contexts, while sustained energy balance and weight stability tend to normalize it. The mechanism is metabolic, not magical.
Exercise
Regular movement tunes insulin receptors and reduces visceral fat. That shift signals the liver to produce more SHBG, often easing an elevated FAI when insulin resistance is in the mix. Short-term, intense sessions can transiently bump testosterone. Long-term, consistent training improves the whole axis — hypothalamus to gonads — toward steadier androgen availability and better recovery. The win comes from repetition and recovery, not a single heroic workout.
Sleep and Stress
Testosterone follows the clock. Most of it peaks in the morning, especially in men. Cut sleep for a week and you’ll likely see that peak shrink. Chronic stress elevates cortisol, which can dampen the brain’s signals to make sex hormones. Improve sleep regularity and stress buffering and you often stabilize the rhythm. The mechanism lives in the hypothalamic-pituitary-gonadal axis and the liver’s SHBG production responding to metabolic cues.
Micronutrients and Supports
Thyroid status strongly influences SHBG. Iodine sufficiency and overall thyroid health, managed with a clinician when needed, can normalize SHBG and with it FAI. Zinc deficiency can impair androgen production; iron deficiency can impair thyroid and mitochondrial function. Vitamin D’s role in androgen biology is still being studied, with mixed findings. If you supplement biotin for hair or nails, know that high doses can interfere with some immunoassays — labs often advise pausing biotin before blood draws to avoid misleading results. Test, adjust, retest, and work with a professional rather than guessing.
Medical Considerations
Medications matter. Estrogen-containing contraceptives and menopausal hormone therapy commonly raise SHBG, lowering FAI. Androgens and some anabolic agents lower SHBG, raising FAI. Thyroid treatment shifts SHBG up or down depending on dose and direction. Metabolic drugs that improve insulin sensitivity can lift SHBG over time. Liver disease, pregnancy, menopause, and significant weight change all reshape this landscape. Major guidelines for PCOS and hypogonadism recommend high-quality testosterone measurement and, when needed, direct free testosterone by equilibrium dialysis or calculated free testosterone using albumin and SHBG. FAI is a helpful screen in women but not the final word.
Connecting the Dots with Related Biomarkers
Total Testosterone is the starting point. In women, low concentrations challenge many immunoassays, so LC-MS/MS is preferred for accuracy. SHBG is the throttle set by your liver and hormones. Together, they generate FAI and help explain why a person with “normal” total testosterone might experience high or low androgen effects.
DHEA-S points to the adrenal side of the story. If FAI is high and DHEA-S runs high too, the adrenal glands may be contributing to excess androgen activity. If DHEA-S is steady and FAI is driven by low SHBG, look harder at metabolic signals and thyroid status.
Albumin helps with calculated free testosterone, which many clinicians prefer over FAI. It uses physics-based binding equations to estimate free hormone more precisely when SHBG is unusual. If the lab provides calculated free T, consider that your primary compass and FAI your quick-glance map.
TSH and thyroid hormones explain a lot of SHBG movement. Hyperthyroidism often raises SHBG; hypothyroidism lowers it. If FAI doesn’t match symptoms, checking thyroid status can clear the fog.
Why Testing Is Worth It
Hormones are dynamic. They respond to sleep, training, food patterns, medication changes, and life phases. One measurement is a snapshot. A few measurements create a time-lapse. That time-lapse lets you see whether a new training block, a change in contraception, or a tighter sleep routine is actually shifting your biology. Prevention and performance both live in the trend lines.
Match numbers with how you feel and function. Energy. Cycle regularity. Skin and hair changes. Recovery. Mood. When the data and your day-to-day align, you get clarity. When they don’t, you know where to look next.
How Superpower Can Help
A smart biomarker panel turns scattered puzzle pieces into a picture you can use. Pair FAI with high-quality testosterone, SHBG, and a few targeted markers, and you move beyond averages toward decisions that fit your body, your goals, and your life. The real win isn’t chasing a perfect number. It’s building a clear feedback loop, grounded in evidence and guided by qualified professionals, so you can act with confidence and course-correct early. Ready to turn your data into direction?
