The Free Androgen Index Calculator calculates the Free Androgen Index from total testosterone and SHBG, aiding assessment of androgen status in adults.
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About the Free Androgen Index Calculator
The calculator estimates an index of free, or unbound, androgens by comparing total testosterone to SHBG. SHBG is a blood protein that binds steroid hormones, including testosterone. When SHBG is high, less testosterone is free; when SHBG is low, more is free. The FAI captures this relationship with a simple ratio.
Healthcare teams often consider FAI when evaluating symptoms linked to androgen imbalance. In women, a higher FAI may support an assessment for hyperandrogenism or polycystic ovary syndrome. In men, a lower FAI can appear when SHBG is raised by age, liver disease, or thyroid conditions. The calculator provides a quick number, but it is not a diagnosis on its own.
How the Free Androgen Index Method Works
Most circulating testosterone is bound to proteins, mainly SHBG and albumin. Only a small fraction is free and biologically active. The FAI estimates this free portion by adjusting total testosterone for SHBG levels. It is a screening method that is simple and fast.
- SHBG binds testosterone with high affinity, reducing free hormone availability.
- FAI increases when total testosterone is high or when SHBG is low.
- FAI decreases when total testosterone is low or when SHBG is high.
- Many conditions change SHBG: thyroid disorders, liver disease, estrogen use, obesity, and insulin resistance.
- FAI is unitless but depends on consistent units for inputs (usually nmol/L).
Because the FAI is a ratio, any error in either input affects the result. Large shifts in SHBG can push the index up or down even if total testosterone is unchanged. For deeper assessment, some clinicians use equilibrium dialysis or validated free testosterone calculations.
Formulas for Free Androgen Index
The standard FAI is derived by comparing total testosterone to SHBG. The method assumes both are measured in the same molar units. Most laboratories report testosterone in nmol/L outside the United States, and ng/dL inside the United States.
- Core formula: FAI = 100 × (Total Testosterone / SHBG), with both in nmol/L.
- If total testosterone is in ng/dL, convert first: nmol/L = ng/dL × 0.0347.
- If you have nmol/L but need ng/dL: ng/dL = nmol/L ÷ 0.0347.
- FAI is unitless; it reflects a ratio, not an absolute hormone concentration.
Many labs also offer calculated free testosterone using albumin and SHBG via validated equations. That approach is different from FAI and can give a more direct estimate of unbound hormone. Use the same unit system across all inputs to avoid errors.
What You Need to Use the Free Androgen Index Calculator
Gather your lab values and a few details before you start. The calculator needs the hormone measurements and their units. Context such as age, sex, and sampling time helps with interpretation, targets, and ranges.
- Total testosterone value (with units: nmol/L or ng/dL).
- SHBG value (nmol/L).
- Units confirmation for testosterone, so conversions are correct.
- Biological sex and age, for context and comparison to lab ranges.
- Sampling time (morning vs later), if available.
Reference ranges vary by laboratory and method. Results can shift with illness, medications, and hormonal therapies. Extreme SHBG values may distort the index, so use caution with edge cases and consider repeat testing.
Using the Free Androgen Index Calculator: A Walkthrough
Here’s a concise overview before we dive into the key points:
- Enter your total testosterone result and choose its unit.
- Enter your SHBG value in nmol/L.
- Confirm that the date and time of sampling are noted for context.
- Select your biological sex and age group.
- Review the calculated FAI displayed on the results screen.
- Compare the result to your lab’s reference range notes.
These points provide quick orientation—use them alongside the full explanations in this page.
Case Studies
A 28-year-old woman reports irregular cycles and acne. Her total testosterone is 1.8 nmol/L, and SHBG is 18 nmol/L. FAI = 100 × (1.8 / 18) = 10. This value is higher than many targets used for premenopausal women and supports evaluating hyperandrogenism with a clinician. What this means: the index suggests increased free androgen activity, but a full workup is needed.
A 55-year-old man has fatigue and low libido. His total testosterone is 10 nmol/L and SHBG is 75 nmol/L. FAI = 100 × (10 / 75) ≈ 13.3. Even with midrange total testosterone, the high SHBG reduces the index, which may correlate with symptoms. What this means: consider repeat morning testing, review medications and thyroid status, and discuss next steps with a clinician.
Limits of the Free Androgen Index Approach
FAI is a screening ratio, not a direct measurement of free testosterone. It works best when SHBG is within typical ranges and the assays are reliable. Results should be interpreted with clinical history and additional tests.
- SHBG is affected by thyroid disease, liver disorders, pregnancy, estrogens, and some medications.
- Acute illness and weight changes can alter SHBG and shift the FAI.
- Immunoassay variability can affect total testosterone and SHBG values.
- FAI may be less reliable in adolescents, pregnancy, or extreme SHBG states.
- Confirmed diagnosis often requires repeat testing and broader hormone panels.
If the FAI conflicts with symptoms, consider measured free testosterone by equilibrium dialysis or a validated calculated method using SHBG and albumin. Use the index as part of a broader evaluation, not a standalone decision.
Units and Symbols
Units matter because the FAI formula requires testosterone and SHBG in the same molar units. Most SHBG results are in nmol/L. Total testosterone may be reported in nmol/L or ng/dL, so conversion is often necessary before calculating the index.
| Item | Symbol | Usual unit | Alternate unit | Notes / Conversion |
|---|---|---|---|---|
| Total testosterone | TT | nmol/L | ng/dL | nmol/L = ng/dL × 0.0347; ng/dL = nmol/L ÷ 0.0347 |
| SHBG | SHBG | nmol/L | — | Protein that binds sex steroids with high affinity |
| FAI | FAI | — | — | FAI = 100 × (TT / SHBG); unitless ratio |
| Albumin | Alb | g/L | g/dL | Used in alternative free T calculations (not for FAI) |
| Sampling time | — | Morning (7–11) | — | Helps reduce diurnal variation in testosterone |
Use the table to check your units before you calculate. If your testosterone is in ng/dL, convert to nmol/L first. Keep SHBG in nmol/L. Then apply the formula to obtain a unitless FAI.
Tips If Results Look Off
If your FAI seems too high or too low, review the basics first. Many errors come from unit mix-ups or timing. Repeat tests are common when early results do not fit symptoms.
- Confirm testosterone units and convert if needed.
- Use a morning blood sample when possible.
- Check if you were ill or on new medications at the time of testing.
- Review thyroid, liver, and estrogen status that may change SHBG.
- Ask the lab about assay methods and reference ranges.
If issues persist, consider a calculated free testosterone method using albumin and SHBG, or a lab that measures free testosterone by equilibrium dialysis. Discuss next steps and targets with your clinician.
FAQ about Free Androgen Index Calculator
What is the Free Androgen Index?
The FAI is a ratio that estimates free testosterone by comparing total testosterone to SHBG. It is quick to compute and helps screen for androgen excess or deficiency.
Is FAI the same as free testosterone?
No. FAI is an estimate based on a ratio. Free testosterone can be measured directly or calculated with equations that include albumin and SHBG.
Can I use FAI while on birth control or hormone therapy?
Hormonal contraceptives and estrogen therapy often raise SHBG, which can lower the FAI. The result may not reflect your baseline status; discuss timing with your clinician.
What FAI value confirms PCOS or low testosterone?
No single FAI value confirms a diagnosis. Laboratories use different ranges, and clinicians combine results with symptoms, exam findings, and other tests.
Glossary for Free Androgen Index
Androgen
A group of hormones, including testosterone, that support male traits and many metabolic functions in all sexes.
Testosterone
The primary androgen. Circulates bound to proteins and in a small free fraction that is biologically active.
Sex Hormone–Binding Globulin (SHBG)
A liver-produced protein that binds testosterone and estradiol with high affinity, reducing the free hormone fraction.
Free Testosterone
The unbound portion of testosterone available to tissues. It can be measured directly or estimated by calculation.
Free Androgen Index (FAI)
A unitless ratio calculated as 100 times total testosterone divided by SHBG, both in nmol/L.
Hyperandrogenism
A state of excess androgen activity. It may present with acne, hirsutism, or menstrual changes in women.
Vermeulen Calculation
A validated method to estimate free testosterone using total testosterone, SHBG, and albumin concentrations.
Polycystic Ovary Syndrome (PCOS)
A common endocrine condition in women, often involving hyperandrogenism, irregular ovulation, and polycystic ovarian morphology.
Sources & Further Reading
Here’s a concise overview before we dive into the key points:
- Vermeulen A et al., A critical evaluation of simple methods for the estimation of free testosterone
- Endocrine Society Clinical Practice Guideline: Testosterone Therapy in Men With Hypogonadism (2018)
- NHS: Sex hormone–binding globulin (SHBG) test overview
- Royal College of Pathologists of Australasia Manual: SHBG
- International evidence-based guideline for the assessment and management of PCOS
- ACOG: Polycystic Ovary Syndrome (PCOS) FAQ
These points provide quick orientation—use them alongside the full explanations in this page.
Disclaimer: This tool is for educational estimates. Consider professional advice for decisions.