New Biomarker Published Jul 7, 2026
Free Testosterone
The share of testosterone in blood that is not bound to proteins.
Also known as
free T · unbound testosterone · free testosterone level · calculated free testosterone · free testosterone direct · free testosterone index · FT
It can explain why hormone symptoms and a total testosterone result do not match.
4 min read · 872 words · 6 sources
In brief
Free testosterone is the unbound fraction of testosterone that can enter tissues, and it matters most when sex hormone binding globulin skews total testosterone or symptoms do not match the total level.
- Free testosterone is the small unbound portion of circulating testosterone, unlike total testosterone that includes bound hormone.6
- Clinicians use free testosterone to clarify low-testosterone questions when total testosterone and symptoms conflict or SHBG is abnormal.43
- Equilibrium dialysis or a validated calculation is more trustworthy than many direct analog free-testosterone immunoassays.5
Deep dive
How it works
SHBG is made mostly in the liver and binds testosterone tightly. Albumin binds testosterone more loosely, so albumin bound testosterone can separate more easily near tissues. That is why some panels report bioavailable testosterone, which usually means free testosterone plus albumin bound testosterone.
When you'll see this
The term in the wild
Scenario
A 42 year old man has total testosterone of 360 ng/dL, free testosterone of 42 pg/mL, and SHBG above the lab range.
What to notice
The total is borderline rather than clearly low, but high SHBG may be holding more testosterone tightly. The free result helps explain why the available share may be low.
Why it matters
This changes the conversation from chasing the total number to finding why SHBG is high and whether symptoms match the lab pattern.
Scenario
A woman being evaluated for irregular periods has free testosterone of 8 pg/mL, while her lab lists a female reference range of 0.1 to 6.4 pg/mL.
What to notice
That is above the example female range, so the clinician would interpret it with symptoms and other tests rather than treating the number alone.
Why it matters
Small absolute changes matter more in women because the usual range is much lower than in men.
Scenario
A supplement label for D-aspartic acid claims it supports testosterone, and a forum user posts only a total testosterone result after taking it.
What to notice
The missing pieces are free testosterone and SHBG. A supplement could leave total testosterone similar while SHBG changes the available fraction, or it could do nothing meaningful.
Why it matters
For supplement claims, free testosterone helps separate a headline hormone number from the amount that is actually available to tissues.
Scenario
A lab report says free testosterone was measured by a direct analog immunoassay, with no SHBG or albumin reported.
What to notice
Some direct methods can be less reliable. A calculated free testosterone using total testosterone, SHBG, and albumin, or equilibrium dialysis, is often more useful when the result will drive a medical decision.
Why it matters
The method can change how much confidence you place in a borderline result.
The full picture
The lab number that can disagree with the headline number
A testosterone report often gives the number everyone recognizes first: total testosterone. That total includes testosterone in three situations. Some is floating unattached. Some is loosely attached to albumin, a common blood protein. Some is tightly attached to sex hormone binding globulin, usually shortened to SHBG. The attached portion still counts in the total, but the tightly held portion is much less available to tissues.
That is why two people can have the same total testosterone and different free testosterone. The surprise is that the total number can look acceptable while the available share is low, especially when SHBG is high. The reverse can also happen when SHBG is low: total testosterone may look low while free testosterone is less abnormal.
What free testosterone actually measures
Free testosterone is the testosterone in blood that is not attached to a carrier protein. It is usually reported as picograms per milliliter (pg/mL), nanograms per deciliter (ng/dL), or sometimes as a percentage of total testosterone. Many commercial adult male reference ranges land roughly around 35 to 155 pg/mL, while adult female ranges are far lower, often around 0.1 to 6.4 pg/mL, but the exact range depends on the lab method.
That lab method matters. Free testosterone can be measured directly by a method called equilibrium dialysis, where the lab separates unbound hormone from protein bound hormone. It can also be calculated from total testosterone, SHBG, and albumin. The Endocrine Society recommends checking free testosterone in men when total testosterone is near the lower limit or when SHBG altering conditions are present, and it specifically favors equilibrium dialysis or a reliable calculation rather than inaccurate analog methods.
When the result deserves attention
Free testosterone is not a general wellness score. For men being evaluated for testosterone deficiency, major guidelines emphasize symptoms plus consistently low testosterone, not one isolated result. Testing is usually most useful in the morning, because testosterone is often highest earlier in the day, and a low result is commonly repeated before decisions are made.
If you are reading this because your free testosterone was flagged, the strongest next step is simple: compare it with total testosterone, SHBG, albumin, the lab reference range, and your symptoms before acting on it. Do not start testosterone or a hormone boosting supplement based only on one free testosterone number. The number is a clue about hormone availability, not a diagnosis by itself.
In women, high free testosterone is often interpreted in the context of symptoms such as acne, increased facial or body hair, irregular periods, or scalp hair thinning. In men, low free testosterone may fit with low libido, fewer morning erections, unexplained anemia, low bone density, or loss of muscle, but those symptoms can also come from sleep loss, depression, medication effects, thyroid problems, alcohol use, or chronic illness.
Myths vs reality
What people get wrong
Myth
Free testosterone is always the best testosterone test.
Reality
It is the best add on test in certain confusing cases, especially when SHBG may be abnormal. Total testosterone is still commonly the starting point.
Why people believe this
Wellness marketing often repeats that free testosterone is the only number that matters, while clinical guidelines use it more selectively.
Myth
A normal total testosterone means free testosterone must be normal too.
Reality
SHBG can hold testosterone tightly, so the total can look acceptable while the free portion is low.
Why people believe this
Most lab reports display total testosterone first, which makes it feel like the main answer rather than one part of the pattern.
Myth
Any flagged free testosterone result proves a hormone disorder.
Reality
A flagged value means the result sits outside that lab's reference range. Diagnosis depends on repeat testing, symptoms, timing, medications, and related labs.
Why people believe this
The named cause is the standard lab flag system: high and low marks are designed to catch out of range values, not to confirm a disease.
Myth
All free testosterone tests are interchangeable.
Reality
Different methods can give different answers. Equilibrium dialysis and reliable calculations are preferred when accuracy matters.
Why people believe this
Reports may all say “free testosterone,” even when the measurement method underneath is different.
Why this keeps coming up
It keeps coming up whenever people are trying to make sense of hormone labs, supplements, and symptom reports that do not line up with the total testosterone number.
How to use this knowledge
A specific failure mode to avoid: do not compare your free testosterone number to a range from another website unless the units and method match. A result in pg/mL cannot be casually compared with ng/dL or percent free testosterone without conversion and context.
What to do with this
- If your total testosterone and symptoms do not match, check free testosterone along with SHBG and albumin.
- Use the lab's own units and reference range, not a range from another site.
- If accuracy matters, prefer equilibrium dialysis or a validated calculated result over a direct analog test.
- Do not make a treatment decision from one flagged result alone.
Frequently asked
Common questions
When should free testosterone be checked instead of only total testosterone?
What time of day should testosterone labs be drawn?
Can supplements raise free testosterone?
Why did my free testosterone change when my total testosterone barely moved?
Is low free testosterone relevant for fertility?
Sources
- 1. Testosterone Therapy for Hypogonadism Guideline Resources (2018)
- 2. Testosterone Levels Test (2025)
- 3. SHBG Blood Test (2026)
- 4. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline (2018)
- 5. Testosterone, Free, Bioavailable and Total, Males (Adult), Immunoassay
- 6. Free Testosterone