FAI%
Typical optimal range
Optimal range varies by lab and individual context.
There is no single universally agreed reference range for FAI, as optimal values differ markedly between sexes and are laboratory-specific; always interpret your result against the reference interval provided by the laboratory that processed your sample. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Interpretation of FAI in the context of PCOS follows NICE guideline NG88 (Polycystic ovary syndrome), and androgen testing more broadly follows NHS and laboratory medicine guidance on sex hormone interpretation.
What is FAI?
The Free Androgen Index (FAI) is a calculated ratio — typically total testosterone divided by sex hormone-binding globulin (SHBG), multiplied by 100 — that estimates the proportion of testosterone circulating freely in the blood and available to body tissues. Because most testosterone is tightly bound to SHBG and therefore biologically inactive, FAI gives a more practical picture of androgenic activity than a total testosterone result alone. It is particularly useful in assessing conditions related to androgen excess or deficiency, and is commonly requested for women being investigated for polycystic ovary syndrome (PCOS) or hirsutism, and for men where androgen status is in question.
What a high FAI means
A raised FAI suggests that a relatively high proportion of testosterone is biologically active, which can be a useful signal that androgen excess may be contributing to symptoms — though it is not a diagnosis on its own.
- Polycystic ovary syndrome (PCOS). PCOS is the most common cause of elevated FAI in women. Insulin resistance associated with PCOS reduces SHBG production in the liver, increasing the free fraction of testosterone and driving up FAI.
- Obesity and insulin resistance. Excess body fat — particularly around the abdomen — suppresses SHBG, raising FAI even when total testosterone is not markedly elevated. This applies to both men and women and can be a finding independent of any underlying hormonal condition.
- Congenital adrenal hyperplasia or androgen-secreting tumours. Less commonly, elevated FAI can result from excess androgen production by the adrenal glands or, rarely, an ovarian or adrenal tumour — conditions a GP would want to rule out if FAI is significantly raised.
A mildly elevated FAI should always be interpreted alongside symptoms, total testosterone, SHBG, LH, FSH, and other relevant markers; a single result without clinical context has limited meaning.
What a low FAI means
A low FAI indicates that a smaller proportion of testosterone is freely available to tissues, which may be relevant in the context of symptoms such as fatigue, low libido, or loss of muscle mass — particularly in men.
- High SHBG. The most common reason for a low FAI is elevated SHBG, which binds more testosterone and reduces the free fraction. SHBG rises with age, thyroid overactivity, liver disease, and certain medications including oestrogen-containing contraceptives.
- Low total testosterone. In men, primary or secondary hypogonadism will reduce both total testosterone and FAI. This warrants further investigation, typically including LH, FSH, and prolactin, as guided by a GP or endocrinologist.
In women, a low FAI is not generally considered clinically significant in isolation and is rarely a cause for concern without accompanying symptoms.
Should I be worried?
A single FAI result outside the reference range is rarely cause for alarm on its own — hormones fluctuate, and a result is always interpreted alongside your symptoms, sex, age, and other blood markers. If your FAI is raised and you have symptoms such as irregular periods, unwanted hair growth, or acne, it is worth discussing with your GP, who may want to repeat the test and check SHBG, total testosterone, LH, FSH, and blood glucose or insulin resistance markers. If FAI is low and you are experiencing symptoms of androgen deficiency, again your GP is the right first point of contact — they can assess whether further investigation or referral to an endocrinologist is appropriate. Try not to read too much into one number in isolation; patterns across repeat tests and the full clinical picture matter far more.
What actually helps
Because FAI is heavily influenced by SHBG levels and underlying metabolic health, several lifestyle factors can meaningfully shift the balance.
- Weight management. Reducing excess body weight — even modestly — raises SHBG and lowers free androgen levels in people with obesity-related androgen excess. A 5–10% reduction in body weight has been shown to improve hormonal profiles in women with PCOS.
- Improving insulin sensitivity. Reducing refined carbohydrate and sugar intake and increasing physical activity both improve insulin sensitivity, which in turn raises SHBG and can lower FAI. This is particularly relevant for people with PCOS or metabolic syndrome.
- Regular physical activity. Both aerobic exercise and resistance training support insulin sensitivity and healthy body composition. Current NHS guidance recommends at least 150 minutes of moderate-intensity activity per week for adults.
- Limiting alcohol. Alcohol can impair liver function and alter sex hormone metabolism; reducing intake supports healthier SHBG production and overall hormonal balance.
There is good supporting evidence that certain dietary patterns and foods can positively influence SHBG and androgen balance.
High-fibre vegetables and legumes
Dietary fibre improves insulin sensitivity and is associated with higher SHBG levels, helping to reduce the free androgen fraction.
Oily fish (salmon, mackerel, sardines)
Omega-3 fatty acids support insulin sensitivity and have anti-inflammatory effects relevant to conditions like PCOS that drive androgen excess.
Whole grains
Replacing refined carbohydrates with whole grains helps moderate blood glucose and insulin spikes, which can otherwise suppress SHBG production.
Soya and flaxseed
These contain phyto-oestrogens that may modestly raise SHBG in some studies, though evidence in humans is mixed and effects are likely small.
Nuts and seeds
Rich in unsaturated fats and magnesium, both of which are associated with improved insulin sensitivity and healthier hormonal profiles.
Green tea
Some evidence suggests green tea polyphenols may modestly raise SHBG and reduce androgen activity, though human data remain preliminary.
Track your own FAI
Upload a lab PDF and Better Days trends FAI over time, flags it when it drifts, and suggests foods that help.
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Sources & further reading
These are reputable UK health resources for further reading. This page is written to be consistent with their guidance but is not a substitute for them.
For information only — not medical advice, and not individually reviewed by a clinician. Reference ranges vary by lab and individual. Discuss any results with a qualified clinician before making changes.