Free Androgen Index (FAI) Blood Test: Meaning and Levels
The free androgen index (FAI) estimates how much testosterone in your blood is biologically active. Normal ranges are 30–150% in men and 0.5–6.5% in women. High FAI levels suggest excess active androgens, often linked to PCOS or insulin resistance. Low FAI levels indicate reduced available testosterone, which may affect energy, libido, and metabolism.
Contents
Result interpretation FAI
What your result means based on common reference ranges
FAI levels within range indicate balanced androgen activity and proper hormone transport, supporting normal metabolic and reproductive function.
Mildly elevated FAI levels suggest early insulin resistance or metabolic imbalance affecting SHBG and androgen regulation.
Significantly high FAI levels indicate androgen excess, commonly linked to PCOS, adrenal disorders, or severe insulin resistance.
Low FAI levels reflect reduced active testosterone, often associated with hypogonadism, aging, or elevated SHBG levels.
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What is FAI?
The Free Androgen Index (FAI) is a calculated value derived from total testosterone and Sex Hormone Binding Globulin (SHBG) levels in a blood test. It reflects the proportion of testosterone that is not tightly bound and therefore biologically active. This index is particularly useful in evaluating hormonal imbalances such as androgen excess, hypogonadism, or insulin resistance. In clinical practice, FAI helps distinguish whether symptoms are due to altered hormone production or changes in transport proteins like SHBG. It is widely used in assessing conditions like PCOS, metabolic syndrome, and endocrine dysfunction.
Reference ranges
Causes of abnormal values
Causes of low FAI
- • Hypogonadism: reduced testosterone production lowers FAI levels
- • Oral contraceptive use: increases SHBG, reducing free testosterone
- • Aging: natural decline in androgen production decreases FAI
- • Hypothyroidism: alters hormone metabolism and may reduce testosterone availability
Causes of high FAI
- • Polycystic Ovary Syndrome (PCOS): increased androgen production and reduced SHBG raise FAI levels
- • Insulin resistance: suppresses SHBG synthesis, increasing free testosterone fraction
- • Obesity: excess adipose tissue alters hormone metabolism and lowers SHBG
- • Adrenal hyperplasia: overproduction of adrenal androgens elevates total testosterone
- • Androgen-secreting tumors: excessive hormone release sharply increases FAI
- • Cushing's syndrome: cortisol imbalance disrupts androgen regulation
FAI in combination with other markers
Possible symptoms
- •Excess facial or body hair (hirsutism)
- •Persistent acne or oily skin
- •Irregular or absent menstrual cycles
- •Decreased libido or sexual dysfunction
- •Thinning scalp hair or male-pattern baldness
- •Weight gain, especially abdominal
When to see a doctor
Consult a doctor if FAI levels are above 10% in women or below 20% in men, especially with symptoms. Seek medical advice for persistent hormonal symptoms, infertility, or rapid physical changes.
What to do if abnormal
- Review medications affecting hormones with your doctor
- Improve diet to reduce insulin resistance
- Increase physical activity to support hormone balance
- Repeat blood test under consistent conditions
- Monitor related markers like SHBG and testosterone
The information on this page is intended for general understanding of test results and does not replace medical consultation. Accurate interpretation requires considering other blood markers and individual factors.
Content is based on medical literature and used within the PulseBook analytical system.
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