T Nation

Free T or Bioavailable?


Quick Q,
what is most important when raising low T, is it the free t figure or the bioavailable figure and what is the diff if any?

I found an online calculator that gives both above figures when you input your total t, shbg and albumin.
So whats bioavailable



You really don't need a calculator for that. Free testosterone = total testosterone minus SHBG-bound testosterone minus albumin-bound testosterone (or just test for free testosterone). Bioavailable testosterone = total testosterone minus SHBG-bound testosterone.

Here's the way it works:
- Free testosterone is very readily available to be used by your body.
- Albumin-bound testosterone is moderately available to be used by your body. It is weakly bound to albumin, so your body needs to break down that bond in order to use the testosterone.
- SHBG-bound testosterone is very weakly available to be used by your body. It is strongly bound to SHBG (sex hormone binding globulin), and your body cannot break that bond very easily, so SHBG-bound testosterone does not do much for you.

Bioavailable testosterone is the medical term for free + albumin-bound testosterone. 'Bioavailable' means that it is the testosterone that your body can use.


FT is T that is not bound to SHBG or other proteins.

SHBG: http://en.wikipedia.org/wiki/Sex_hormone-binding_globulin
Note that SHBG does not transport and release T, SHBG bound [SHBG-T] is not bioavailable. You will find many incorrect references to the contrary.

SHBG-T is tightly bound. Estrogens bind weakly to SHBG and SHBG can transport estrogens and release the estrogens throughout the body. Note that SHBG occurs in blood, but does not exist in tissues. So all T in tissues is considered FT. Secretions, such as saliva, can be used for hormone lab work and the results are considered FT, thus there is no TT in saliva testing. Many feel that saliva testing is quite useless.

Weakly bound T: T can be bound to other proteins such as albumin in the blood. Those are weakly bound and the albumin bound T can be release T to tissues and this is considered hormone transport. There are other proteins to which T can be weakly bound. Note that some males loose serum albumin levels as they age. This is thought to also reduce bio-T:TT to some extent. In reality, these guys have low T, and lower albumin is one of the catabolic effects of low T. TRT would be expected to improve albumin levels.

With age, we see E2 and SHBG increase. This tends to increase TT and that can be misleading.

Bio-T is FT plus weakly bound T. Both measures are technically useful. Bio-T and FT are roughly proportional. So you typically do not have any need to do both of these types of labs.

TT is FT + weakly bound + SHBG-T. Note that for a given production rate of T, or a given TRT dose of T, the more SHBG you have, the more TT you will have. In this regard, TT can be the wrong therapeutic target and can be a simplistic and inappropriate concern if one does not understand the larger picture.

From a practical point of view, lab results should be useful. So the results need to be something that others are sufficiently familiar with to interpret. Most here are familiar with FT and not with bio-T. So FT will be better from that point of view.

To put the whole picture together, you need estradiol numbers. With FT, TT and E2, there is very little need for SHBG lab numbers.

Note that some labs produce high ranges and results for FT compared to other labs. FT lab numbers always need the lab ranges for interpretation. In this regard, I expect that bio-T has the same problem and bio-T results will always needs the lab's ranges for interpretation.

In HPTA intact males, LH is released in pulses and diurnal patterns. This leads to changing FT levels. FT has a short half life. When you test FT in these males, the absolute number is not very important as the result is partly a factor of when the lab work was done. With TRT using transdermals, the swings in FT are more extreme and lab timing greatly determines the lab values. With frequently injected T, TT, FT and E2 levels can be very steady and FT lab results thus are more valuable in that case.


Im confused could it be explained in an easier way to keep it simple, is free T and bioavailable T more or less the same thing, or is one of the figures more important than the other such as free?

here in the uk docs onlytestfor total and calculate free t from shbg, they do not test for free T as a stand alone test.
Its beacause i came across a calculator where you input total T, shbg and albumin (these figures which i have from blood tests), and it gives the results of free t and bioavailble t (of which i never heard of and only familiar with free t), thats why i asked what is the more important figure free or bioavailable?


there isn't really a way to make it less complex. This is about as simple as it gets. HRT, hormone balance, system interaction, hormone pathways, ezyme disruptions, etc. are extremely complicated. Heck most doctors don't understand half of the stuff they are prescribing and they can't even interprete blood test results correctly.

both free T and bioavailable T are useful numbers (depending on who you ask). I don't know if anyone can say that one is 4.67x as important as the other.


The UK docs that you're talking about are testing bioavailable testosterone - i.e. total testosterone minus SHBG-bound testosterone. This is a useful measure. Personally, I'd prefer to see both bioavailable and free, but they generally go hand-in-hand - high bioavailable T will usually mean you have high free T, and vice-versa.

So, to answer your question: neither is more important. They are usually inextricably linked to each other, so a deficiency in one usually means that there's a deficiency in the other; likewise, an increase in one usually means an increase in the other.


I understand, the uk nhs only test for free t which is done by calculating from total t and shbg not a specific test for free, and there is no word or mention of bioavailable again on the nhs.
I do know that just because total has raised it doesnt men free t has raised automatically.