T Nation

TT vs FT, Some Confusion


#1

I’ve been reading some conflicting information on total testosterone and free testosterone.

According to William’s Textbook of Endocrinology 12th ed., bound testosterone is composed of SHBG-bound and Albumin-bound. The text book states that bound T is about 30 - 44% SHBG and 54 - 68% albumin. It also goes on to say that albumin-bound is “bio-available” and useful to the system.

That statement would mean that FT is only part of the equation and that we’d need to monitor the albumin-bound to really track our true testosterone. That idea seems to be contrary to what I’ve read in the stickies.

Things, however, get a little more confusing when you go to Labcorp’s website. I was on there reading about blood test and they have the following statement, "the majority (approximately 60% to 90%) of serum total testosterone is associated with sex hormone binding globin (SHBG); this fraction is tightly bound and biologically unavailable to its target tissues. The remaining bioavailable testosterone is mostly bound to albumin, with only a small fraction (approximately 0.5% to 2%) circulating in the free form". Labcorp and William’s disagree by quite a bit on the amount of T bound to SHBG. They agree, roughly, on the amount of FT as a percentage.

Any insights into these discrepancies? Does albumin-bound T make up a small portion of bound-T?

Link to Labcorp’s test: https://www.labcorp.com/wps/portal/!ut/p/c1/04_SB8K8xLLM9MSSzPy8xBz9CP0os_hACzO_QCM_IwMLXyM3AyNjMycDU2dXQwN3M6B8JG55AwMCuv088nNT9SP1o8zjQ11Ngg09LY0N_N2DjQw8g439TfyM_MzMLAz0Q_QjnYCKIvEqKsiNKDfUDVQEAOrk-dE!/dl2/d1/L0lJWmltbUEhL3dQRUJGUUFoTlFBaERhQUVBWEtHL1lJNXlsdyEhLzdfVUU0UzFJOTMwT0dTMjBJUzNPNE4yTjY2ODAvdmlld1Rlc3Q!/?testId=6305022#7_UE4S1I930OGS20IS3O4N2N6680

Link to William’s Textbook: https://www.amazon.com/Williams-Textbook-Endocrinology-Shlomo-Melmed/dp/1437703240/ref=sr_1_3?s=books&ie=UTF8&qid=1485794226&sr=1-3&keywords=williams+textbook+of+endocrinology


#2

You can get Bio-T tested [FT and albumin+T]

William’s Textbook of Endocrinology is taking about fractions within bound T fraction.
Labcorp is talking fractions of TT. Different references.

Conflicts: Depends on your albumin levels and SHBG levels. We see very large variations in SHBG lab results here. It does not matter where those two generalities lie. With your understanding, your lab work is all that you need as you are a sample population of one.

If FT is higher, Bio-T will be higher and they generally track together. So there typically is no need to check both. If FT is high, we can assume that SHBG is lower so there generally is no need to check FT and SHBG.


#3

Thanks for the response, and I apologize that I didn’t have the direct quote from William’s.

Williams Textbook of Endocrinology, page 703 says, “In the circulation, total testosterone is composed of 0.5% to 3.0% free testosterone unbound to plasma proteins, 30% to 44% SHBG-bound testosterone, and 54% to 68% albumin-bound testosterone”. I read that as William’s saying that SHBG is 30-44% of TT, not bound-T. It still seems to contradict Labcorp’s “the majority (approximately 60% to 90%) of serum total testosterone is associated with sex hormone binding globin (SHBG)”.

There is a newer addition of Williams Textbook of Edocrinology, which might have different numbers. If so, I apologize. It may be possible that the accepted values have changed in the last four years. Does anyone have another source that states the amount of SHBG-bound T?


#4

Probably boils down to what sample group is used and not all lab companies produce the same results as see with vastly differing FT ranges.


#5

That very well may be it. I like your previous point the FT and albumin-bound follow the same trend, so tracking one indirectly tracks the other.