Biomarker
robust evidence
4 min · 6 sources
Published Jul 7, 2026

Free Testosterone

The share of testosterone in blood that is not bound to proteins.

It can explain why hormone symptoms and a total testosterone result do not match.

Also known as free T · unbound testosterone · free testosterone level · calculated free testosterone · free testosterone direct · free testosterone index · FT

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

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.

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

A 42 year old man has total testosterone of 360 ng/dL, free testosterone of 42 pg/mL, and SHBG above the lab range.

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.

This changes the conversation from chasing the total number to finding why SHBG is high and whether symptoms match the lab pattern.
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.

That is above the example female range, so the clinician would interpret it with symptoms and other tests rather than treating the number alone.

Small absolute changes matter more in women because the usual range is much lower than in men.
A supplement label for D-aspartic acid claims it supports testosterone, and a forum user posts only a total testosterone result after taking it.

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.

For supplement claims, free testosterone helps separate a headline hormone number from the amount that is actually available to tissues.
A lab report says free testosterone was measured by a direct analog immunoassay, with no SHBG or albumin reported.

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.

The method can change how much confidence you place in a borderline result.

Myths vs reality

What people get wrong

Myth

Free testosterone is always the best testosterone test.

Why people believe it Wellness marketing often repeats that free testosterone is the only number that matters, while clinical guidelines use it more selectively.

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.
Myth

A normal total testosterone means free testosterone must be normal too.

Why people believe it Most lab reports display total testosterone first, which makes it feel like the main answer rather than one part of the pattern.

Reality

SHBG can hold testosterone tightly, so the total can look acceptable while the free portion is low.
Myth

Any flagged free testosterone result proves a hormone disorder.

Why people believe it 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.

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.
Myth

All free testosterone tests are interchangeable.

Why people believe it Reports may all say “free testosterone,” even when the measurement method underneath is different.

Reality

Different methods can give different answers. Equilibrium dialysis and reliable calculations are preferred when accuracy matters.

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.

Frequently asked

Common questions

When should free testosterone be checked instead of only total testosterone?

It is most useful when total testosterone is borderline, symptoms and total testosterone do not match, or SHBG may be abnormal because of age, weight change, thyroid disease, liver disease, medications, or other conditions.

What time of day should testosterone labs be drawn?

For most adult men being evaluated for low testosterone, morning testing is usually preferred because testosterone often runs higher earlier in the day. A low value is commonly repeated before treatment decisions.

Can supplements raise free testosterone?

Some supplements are marketed for testosterone support, but a claim on a label is not proof that free testosterone will rise meaningfully. If you are tracking a supplement, compare morning labs using the same lab and include SHBG, not just total testosterone.

Why did my free testosterone change when my total testosterone barely moved?

SHBG may have changed. When the binding protein rises or falls, the free share can shift even if the total amount changes little.

Is low free testosterone relevant for fertility?

It can be part of the hormone picture, but starting testosterone can reduce sperm production. Anyone trying to conceive should discuss results with a clinician before using testosterone therapy.

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