Free Androgen Index (FAI)
Estimates how much of your testosterone is biologically active, not just how much you have in total.
Consider this test if:
- Low libido, fatigue, low mood, or loss of muscle and strength despite a normal total testosterone
- Irregular cycles, acne, or unwanted hair growth as part of a PCOS evaluation
- Conditions that shift SHBG, such as thyroid disease, obesity, insulin resistance, or oral estrogen use
- Setting an androgen baseline or tracking changes during hormone therapy
- HSA/FSA eligible
- Typical results in 1-2 days · Reviewed by a real clinician
- Drawn at a CLIA/CAP-accredited lab near you ·
Pre-test considerations
Draw the sample in the morning, ideally before 10 a.m., when testosterone peaks. Fasting is not required. Acute illness, recent intense exercise, and hormonal medications can shift the result, so note any testosterone or estrogen therapy. Because levels fluctuate, an abnormal result is usually confirmed with a repeat morning draw.
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What this test is for
The Free Androgen Index estimates physiologically active testosterone from two measured values: total testosterone and sex hormone-binding globulin (SHBG), calculated as their ratio times 100. It matters because most circulating testosterone is bound tightly to SHBG and biologically inactive, so total testosterone alone can read normal while active testosterone is low, often when SHBG is high. The index is most useful for investigating symptoms of low androgen activity (low libido, fatigue, low mood, reduced muscle and strength in men) and signs of androgen excess in women (irregular cycles, acne, hirsutism), where it commonly supports a workup for PCOS.
FAI is a calculated estimate that works best when SHBG is in a normal range; at extreme SHBG levels, a directly measured free or bioavailable testosterone is the more reliable choice. If you want the underlying numbers separately, total testosterone and SHBG are also offered on their own.
Biomarkers tested
Includes 3 biomarkers
- Specimen
- Serum or plasma
- Measures
- Ratio
SHBG is a liver-made protein that binds testosterone and estradiol, controlling how much hormone is free to act on tissue rather than locked up in transit. High SHBG (common with high estrogen, hyperthyroidism, or liver disease) can leave you hormonally starved even when total testosterone looks normal, while low SHBG (seen with insulin resistance, obesity, or hypothyroidism) inflates free hormone activity. Pairing SHBG with total testosterone gives you an actual free testosterone picture, useful for investigating low libido, fatigue, mood changes, or irregular cycles, and for tracking how weight loss or metabolic changes are shifting your hormone availability over time.
- Specimen
- Serum or plasma
- Measures
- Substance concentration
Total testosterone measures the hormone your testes (or ovaries and adrenal glands in women) produce to drive libido, muscle mass, bone density, energy, and mood. Low levels show up as fatigue, low sex drive, harder recovery from training, mood changes, and difficulty building or keeping muscle, while high levels in women can signal PCOS or point to an androgen-secreting source. It is the standard baseline for anyone tracking hormonal health with age, evaluating symptoms, or checking response to training, weight loss, or testosterone therapy.
- Specimen
- Serum or plasma
- Measures
- Mass concentration
What you'll learn
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Insights that connect the dots. A clinician reads your whole picture, not one result in isolation, connecting family history, past results, and lifestyle into clear next steps.
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Your personalized action plan. Nutrition, supplements, and training in one clinician-reviewed protocol, updated as your results change.
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Track your trends. Upload results from any lab and see each biomarker trend over time, so you know what is working.
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Never miss a recheck. Mito tells you what is worth retesting and when, then reminds you so nothing slips.
What to expect
- 1 Book instantly
Click, book, done. Choose a convenient lab location near you. Transparent, up-front pricing.
- 2 Quick lab visit
Testing to fit your busy schedule, usually 15 minutes or less. Walk-in and appointments available.
- 3 Typical results in 1-2 days
Your results post straight to your dashboard as soon as the lab completes them.
- 4 Expert guidance
Included with Mito membership. A clinician reviews your results and your personalized action plan follows, with clear next steps.
Everything your health needs,
in one membership
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Every test at our cost
Members pay our cost on every test, with lab fees passed straight through. The full receipt is itemized, never padded.
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Continuous tracking, all in one place
Upload past labs and watch your trends over time. Every marker and visit lives in one longitudinal record, so all your care stays together.
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Year-round clinician support
Text anytime and get clinician-reviewed answers. When you want to go deeper, 1:1 consultations are available at affordable rates.
All for $9/month
Order any test or consult without joining. For $9/month, members unlock member prices, trend tracking, and year-round clinician guidance.
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Frequently asked questions
View all FAQsHow does pricing work?
Every test shows the member price next to the standard non-member price, so you can see what membership saves you. The member price is our cost, covering the lab and what it takes to run the service. It is never a profit on the test itself. Mito makes its money on the $9 membership, not on marking up your tests. Membership is $9 per month, and any applicable lab order or draw fee is itemized before you pay.
Where do I get tested?
Choose an available partner lab at checkout. If your cart contains multiple blood tests, Mito helps consolidate the eligible tests at one lab when possible. At-home kits and scans follow the collection or appointment process shown on their product page.
Is this eligible for HSA/FSA?
Yes. Eligible tests can be paid for with an HSA or FSA card at checkout.
When will I get my results?
Your results appear in your Mito dashboard after the laboratory completes processing. A clinician then reviews them before your analysis and personalized action plan are finalized. Timing varies by test, and specialty assays and at-home kits may take longer.
Do I need a physician's order?
No. Mito provides the laboratory order when one is required, so you do not need to arrange a separate physician visit before purchasing.
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