Bioavailable Testosterone
Free Plus Albumin-Bound - Definition and When It Is Actually Used
Evidence grade 4 of 5, Strong. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- A laboratory measurement; measured by ammonium-sulfate precipitation of SHBG or calculated from total T, SHBG, and albumin.
Reference only. Not medical advice.
What can go wrong
Bioavailable T is an extra measurement that rarely alters a decision already answerable from total T and SHBG; its main misuse is as an upsell that generates additional 'below optimal' findings. The direct precipitation method also has its own imprecision, and the calculated version inherits errors from its inputs.
This is something to take to a clinician. The Atlas can tell you what is known about 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.
Bioavailable testosterone is the free fraction plus the loosely albumin-bound fraction - roughly 30-55% of total testosterone - reflecting the pool that can dissociate and reach tissue. It is measured directly by selectively precipitating SHBG-bound testosterone with ammonium sulfate, or calculated from total T, SHBG, and albumin (same Vermeulen framework as free T). It is most useful when SHBG is abnormal, particularly in older men with rising SHBG or in the setting of altered albumin from illness.
The middle ground between total and free - useful, and easy to over-order.
What people take it for
What it is taken for. Approval, where it exists, covers only some of these.
How it works
Albumin binds testosterone with low affinity, so albumin-bound hormone dissociates readily at the capillary and is effectively available to tissue. Bioavailable T therefore captures more of the physiologically usable pool than free T alone. In older men, SHBG rises and total T can stay deceptively normal while bioavailable T declines linearly with age - which is why bioavailable T is often preferred in geriatric assessment.
The catch: three overlapping numbers, one decision
Total, free, and bioavailable testosterone are three views of the same hormone, and no guideline asks for all three routinely. Ordering the full stack rarely changes management beyond what a properly timed total T plus SHBG (and free T when needed) already tells you, but it multiplies the line items on a panel and the surface area for a 'suboptimal' flag. The marketplace tends to order everything and then find something to treat.
What people get wrong
Often saidMore testosterone fractions on the report means a more thorough workup
ActuallyBeyond total T plus SHBG (and free T when indicated), extra fractions rarely change management - they mostly add cost and chances for a 'suboptimal' flag.
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
- Definition
- Free + albumin-bound testosterone
- Fraction of total
- ~30-55%
- Direct method
- Ammonium-sulfate precipitation of SHBG-bound T
- Calculated from
- Total T, SHBG, albumin
- Best use
- Older men, high SHBG, altered albumin
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about 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.
- 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
Bioavailable testosterone declines linearly across the lifespan, Baltimore Longitudinal Study of Aging2016
Showed bioavailable T falls steadily with age, supporting its use over total T in older men
Vermeulen et al., JCEM1999
Provided the calculation for bioavailable as well as free testosterone from total T, SHBG, and albumin
Endocrine Society guideline2018
Endorses free or bioavailable testosterone when total T is borderline or SHBG is altered