Free vs Total vs Bioavailable Testosterone
The Three Numbers, the Active Fraction, and Why They Disagree
Evidence grade 4 of 5, Strong. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- Not applicable (measurement concept)
Reference only. Not medical advice.
This is something to take to a clinician. The Atlas can tell you what is known about free vs total vs bioavailable testosterone in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.
Total testosterone is the sum of all circulating fractions; free testosterone (~2-3%) is immediately bioactive; bioavailable testosterone is free plus the loosely albumin-bound fraction. The three can diverge because binding-protein levels, chiefly SHBG, change the active proportion. Understanding which number matters prevents both over- and under-treatment.
Why a 'normal' total can hide a low active level, and vice versa.
What it is invoked to explain
Claims this idea is used to support. Whether it supports them is the rest of the entry.
How it works
Testosterone circulates bound to SHBG (high affinity, effectively inactive), bound to albumin (low affinity, dissociates readily and is weakly available), or free. Only free and the readily dissociable albumin-bound fraction reach tissues, so bioavailable (free + albumin-bound) and free testosterone are the physiologically meaningful measures. When SHBG is high (aging, hyperthyroidism, liver disease), total can look normal while free is low; when SHBG is low (obesity, insulin resistance, hypothyroidism), total can look low while free is relatively preserved.
Measurement caveats
Direct free-testosterone immunoassays (analog assays) are unreliable. The accepted approaches are equilibrium dialysis (reference method) or calculated free testosterone derived from total testosterone, SHBG, and albumin. A calculated free value is only as good as the total and SHBG inputs, so assay method still matters underneath.
Safety in detail
Interpretation should be confirmed with a qualified clinician. Changes to hormones, sexual function, or sexual health warrant evaluation by a qualified healthcare provider.
What it does to fertility
These fractions describe circulating hormone, not directly fertility; however, the same panel that distinguishes them (with LH and FSH) is used to evaluate the gonadotropin status relevant to spermatogenesis before any therapy.
What has to be monitored
The bloodwork this needs, and the reason it belongs with a clinician rather than a website.
- Total testosterone (morning)
- SHBG
- Albumin (for calculated free testosterone)
- Free testosterone by equilibrium dialysis or validated calculation when SHBG is abnormal
What people get wrong
Often saidFree testosterone is just total testosterone divided by a constant
ActuallyNo, the ratio depends on SHBG and albumin, which vary widely between men and with health conditions. That is exactly why the free and bioavailable fractions are measured or calculated separately.
Who profits, and who pushes back
Nobody is named here yet, because we have not done that work on this entry. Read that as unfinished. Somebody almost certainly sells this, and somebody almost certainly disputes it. Assume both until we have checked.
At a glance
- Free fraction
- ~2-3% (bioactive)
- Albumin-bound
- ~50-55% (weakly available)
- SHBG-bound
- ~40-45% (inactive)
- Bioavailable
- Free + albumin-bound
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about Free vs Total vs Bioavailable Testosterone. A short entry is not a safe entry.
- Timeline. We have not built the dated record of research and regulatory events.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
- What can go wrong. We have not written the risk line for this one. Read the safety section below rather than treating its absence as reassurance.
- Where we land. We have not taken a position on this yet.
Sources
The studies below are named and described, but not linked. You cannot open them from here and check our reading against them, which means this page is our summary of the literature rather than a document you can verify. Weigh it accordingly until the links exist.
The studies that matter
Endocrine Society guideline guidance on free testosterone2018
Recommends measuring or accurately calculating free testosterone when total is near the lower limit or when SHBG abnormalities are suspected, using equilibrium dialysis or validated calculation rather than direct analog immunoassay