Is Bioavailable another word for Free Testosterone?

[SkyWarn]

I have been on T for almost 10 years and I know about free and total T, but for the first time a new doc tested bio-available T. I always though bio-available was another word for free T. My numbers are:

Total 799
Free 196
Bioavailable 511

All 3 are flagged as high by the lab. So what's the significance with bioavailable?

 

[Cataceous]

Bioavailable testosterone is the amount of testosterone not bound to SHBG, which is the sum of free testosterone and albumin-bound testosterone. Bioavailable testosterone is available on a relatively short time scale, on the order of minutes. Whereas it takes considerably longer to free up SHBG-bound testosterone.

 

[MikeXL]

Think of it this way. You have free testosterone and bound testosterone. The bound testosterone is either "tighly" bound by SHBG or "losely" bound by albumin and other binding proteins. Because the testosterone that is bound by proteins other than SHBG is losely bound, it is consider "available". BAT = FT +losely bound T

 

[Vince Carter]

Lightly bound to albumin is still bound, right? Does it become unbound in some way?

 

[MikeXL]

Lightly bound to albumin is still bound, right? Does it become unbound in some way?

Yes. It is currently bound. But it can become unbound. Hence why it is considered "bioavailable"

Also, it still isnt 100% clear that the SHBG bound is completely inert. The notion that bound testosterone never does anything is part of the "free hormone hypothesis". Notice that it is a hypothesis. However, it is easy to understand and so people ran with it.

Greek researchers have suspected that this may be a faultly hypothesis since the early 2000s.

You see, there is a receptor on the cytosol called the megalin receptor. SHBG binds to testosterone in the serum creating a SHBG-Testosterone complex. This SHBG-Testosterone complex can then bind to the megalin receptor. Once SHBG-testosterone-megaline-R is a complex it seems that it can travel to the nucleas and transcribe messages

 

[Vince Carter]

In what scenario, is my real question? I think we generally understand that bound is bound is bound and thus not available, be it Albumin, or SHBG.

But it can become unbound

 

[Cataceous]

In what scenario, is my real question? I think we generally understand that bound is bound is bound and thus not available, be it Albumin, or SHBG.

Spontaneously. Think of a couple monkeys fastened together by velcro. They squirm around and eventually they pull just right and detach from each other. The "velcro" for SHBG is stronger than it is for albumin, so on average it takes longer to get loose.

 

[Vince Carter]

Id like to hear you defend this:

Whereas it takes considerably longer to free up SHBG-bound testosterone.

 

[Cataceous]

Id like to hear you defend this:

This paper finds the testosterone-albumin dissociation rate constant to be 2.1-2.2/second and the testosterone-SHBG dissociation rate constant to be 0.053–0.058/second. I believe this means that on average it takes testosterone about 40 times longer to dissociate from SHBG than from albumin.

 

[madman]

I have been on T for almost 10 years and I know about free and total T, but for the first time a new doc tested bio-available T. I always though bio-available was another word for free T. My numbers are:

Total 799
Free 196
Bioavailable 511

All 3 are flagged as high by the lab. So what's the significance with bioavailable?

Partitioning of testosterone in the systemic circulation. Circulating testosterone is bound tightly to SHBG (green = high affinity binding) and weakly to albumin, orosomucoid (ORM), and CBG (blue = low affinity binding) (11). Only 1% to 4% of circulating testosterone is unbound or free. The combination of free and albumin-bound testosterone is also referred to as the “bioavailable testosterone” fraction.

 

[SkyWarn]

So which is the part that drives libido; bound, free or bio-available?

 

[Cataceous]

So which is the part that drives libido; bound, free or bio-available?

In general free testosterone is doing the useful stuff. But if you're interested in the nitty-gritty, there is also some evidence that SHBG-bound testosterone is used directly in some places, as @MikeXL notes above.