Free Androgen Index FAI

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Ratio of testosterone to SHBG.

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Free Androgen Index (FAI) - Comprehensive Medical Test Guide

Why is it done?
- Test Overview: The Free Androgen Index (FAI) is a calculated ratio that estimates the percentage of free (unbound) testosterone in relation to total testosterone and sex hormone-binding globulin (SHBG). It provides a more accurate assessment of biologically active androgens than total testosterone alone.
- Primary Indications: Evaluation of hyperandrogenism (elevated androgens) in women; Assessment of polycystic ovary syndrome (PCOS); Investigation of hirsutism, acne, or male pattern baldness; Evaluation of infertility in both males and females; Assessment of virilization symptoms; Monitoring of androgen-suppressing therapy
- Typical Timing: Often performed during the follicular phase (days 3-5) of the menstrual cycle in women; Can be ordered as part of initial hormonal workup for reproductive concerns; Used when SHBG abnormalities may affect free testosterone levels; Repeated at intervals to monitor treatment efficacy
Normal Range
- Reference Values: Women (reproductive age): 0.8-3.5 or 1-5 (varies by laboratory); Women (postmenopausal): 0.5-2.0; Men: 24-104 or higher depending on laboratory reference ranges; FAI is calculated using the formula: (Total Testosterone in nmol/L ÷ SHBG in nmol/L) × 100
- Units of Measurement: FAI is expressed as a percentage or unitless ratio; Total testosterone: ng/dL or nmol/L; SHBG: nmol/L or μg/mL
- Result Interpretation: Normal FAI: Indicates appropriate proportion of free androgens; biologically active testosterone is within expected range
- Elevated FAI: Suggests increased free androgens relative to SHBG; May indicate hyperandrogenism even when total testosterone is normal; Associated with clinical symptoms of androgen excess
- Low FAI: Indicates reduced free androgens or elevated SHBG; May suggest hypogonadism in men; Can occur with certain medications or medical conditions
Interpretation
- Clinical Significance: FAI is superior to total testosterone in detecting androgen-related disorders because it accounts for SHBG variation. Only free testosterone exerts biological effects on target tissues.
- Factors Affecting FAI Levels: SHBG levels (inversely proportional); Liver disease (affects SHBG production); Estrogen exposure (increases SHBG); Insulin resistance (decreases SHBG); Thyroid disorders; Medications (oral contraceptives, corticosteroids); Body composition and weight; Polycystic ovary syndrome; Adrenal disorders; Ovarian tumors; Testicular disorders
- In Women with Elevated FAI: May present with hirsutism, acne, male-pattern baldness, irregular menses; Often associated with PCOS; Can indicate ovarian or adrenal androgen-secreting tumors if markedly elevated; May contribute to infertility
- In Men with Abnormal FAI: Low FAI may indicate primary or secondary hypogonadism; High FAI less common but can occur with certain conditions; May affect sexual function, fertility, and bone health
- Calculating FAI: Formula: FAI = (Total Testosterone × 100) ÷ SHBG; This calculation requires concurrent measurement of both total testosterone and SHBG; Most laboratories perform this calculation automatically
Associated Organs
- Primary Organs Involved: Testes (primary testosterone source in men); Ovaries (androgen production in women); Adrenal glands (DHEA and other androgens); Liver (SHBG synthesis); Hypothalamus and pituitary gland (hormonal regulation)
- Conditions Associated with Elevated FAI: Polycystic ovary syndrome (PCOS); Adrenal hyperplasia; Ovarian or adrenal tumors; Insulin resistance and metabolic syndrome; Acromegaly; Cushing's syndrome; Hyperprolactinemia
- Conditions Associated with Reduced FAI: Primary hypogonadism; Secondary hypogonadism; Klinefelter syndrome; Liver cirrhosis; Chronic kidney disease; Hypothyroidism; Medications (antiandrogens, GnRH agonists); Obesity; Aging
- Potential Complications of Abnormal Androgens: Infertility; Irregular or absent menstruation; Sexual dysfunction; Cardiovascular complications; Bone loss (osteoporosis); Metabolic disorders; Psychological effects (depression, anxiety); Increased cancer risk (with severe alterations)
- Target Tissues Affected by Free Androgens: Hair follicles (hirsutism and alopecia); Sebaceous glands (acne and oily skin); Prostate gland; Bone; Muscle; Brain; Cardiovascular system
Follow-up Tests
- Recommended Additional Testing: Total testosterone and SHBG (component values); Luteinizing hormone (LH); Follicle-stimulating hormone (FSH); Prolactin; 17-hydroxyprogesterone (for adrenal hyperplasia); DHEA-S (dehydroepiandrosterone sulfate); Estradiol; Thyroid function tests (TSH, free T4); Fasting glucose and insulin (for insulin resistance); Pelvic ultrasound or pelvic MRI (if tumor suspected); Adrenal imaging if indicated
- For PCOS Diagnosis: Pelvic ultrasound to assess ovarian morphology; Fasting glucose tolerance test; Lipid panel; Blood pressure monitoring; Assessment of menstrual history and clinical symptoms
- For Male Hypogonadism Investigation: LH and FSH; Prolactin level; Semen analysis; Imaging (testicular ultrasound, pituitary MRI); Bone density scan (DEXA); Metabolic assessment
- Monitoring During Treatment: Repeat FAI testing: At 6-8 weeks after starting treatment, then every 3-6 months; Total testosterone and SHBG monitoring; Clinical symptom assessment; Liver and renal function tests (if on medications); Metabolic monitoring
- Complementary Tests: Free testosterone (direct measurement); Bioavailable testosterone; Estrone and estradiol; Androstenedione; Cortisol (if adrenal disease suspected); Growth hormone and IGF-1 (if acromegaly suspected)
Fasting Required?
- Fasting Requirement: No - Fasting is NOT required for Free Androgen Index testing. The test can be performed regardless of food or fluid intake.
- Sample Collection Timing: For women: Ideally collected during follicular phase (days 3-5 of menstrual cycle) for standardization; Morning collection (8 AM-10 AM) preferred, as testosterone levels are higher earlier in the day; For men: Morning collection preferred due to diurnal variation
- Medications to Continue/Discontinue: Continue taking regular medications unless physician instructs otherwise; Inform physician of hormonal medications (oral contraceptives, HRT, antiandrogens); Some medications should be held 24-48 hours before testing per physician direction; Steroid medications may affect results
- Patient Preparation Instructions: Wear comfortable clothing with easily accessible arms; Avoid strenuous exercise 24 hours before testing; Get adequate sleep the night before; Avoid stress if possible; Arrive early to allow for relaxation before blood draw; Bring insurance card and identification; Notify phlebotomist of any needle anxiety
- Sample Type: Serum (blood sample); Typically collected via venipuncture (needle from arm vein); Sample placed in appropriate collection tube; No special handling required for routine transport

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