Free testosterone is typically between 1.5-2.5% of a man's total testosterone. This means, of course, that a man's free testosterone will be about 1/50th of his total, obviously a much smaller number. Perhaps because the number is so much smaller - I don't know for sure - I often see on the The Peak Testosterone Forum that men have clearly inaccurate results. Sometimes men will have free testosterone that is 0.1% of total testosterone, for example. These kind of results are ridiculous and simply make no sense - any man with free testosterone that low would almost for sure be anemic and have osteopenia. In my opinion, the lab should have some check to see if their results are reasonable and mark samples like these invalid. But anyway...
Another possibility is confusion over the units. Labs here in the U.S. often give total testosterone in ng/dl and free testosterone in pg/ml. My guess is that often the patient, doctors or nurse dealing with the results forget are rushing and do not take the change of units into account.
So does this mean that you should not even bother pulling free testosterone? Of course, do whatever your doctor tells you - go ahead and get it pulled. Sometimes the test does pull reasonable results. However, here is from what I have seen a much more reliable way to pull free testosterone:
1. Pull total testosterone and SHBG on the same day. (Make sure that you have the correct units.)
2. Use one of the online free testosterone calculators to calculate free testosterone.
Why does this work so well? The reason is that roughly two thirds of non-free, or bound testosterone is actually chemically bonded to a protein called Sex Hormone-Binding Globulin (SHBG) and rougly 1/3 to a protein called albumin. So to figure out free testosterone, you essentially need SHBG, albumin and total testosterone. However, albumin does not fluctuate much, except in the case of certain medical conditions, and so you can make a reasonable assumption that it is about 4.3 g/dl. And, once you make this assumption, you can use it in to the online calculators, and directly but accurately estimate free testosterone.
On The Peak Testosterone Forum I have seen huge variations in SHBG from one man to the other - from as low as 10 to 85 nmol/l. These kind of variations can have an impact on free testosterone, and this is the reason it is important to pull SHBG: you simply cannot make assumptions about your SHBG levels.
That said, one's SHBG, at least from what I have seen over the years, does not swing widely from lab read to lab read. If you pull your SHBG, it will be the quite similar every time you pull out. I do not recall a single case of a man saying he pulled SHBG and then it varied dramatically the next time he pulled it, which is not true with free testosterone. It seems to be a very stable test.
Here are Two Other Reasons to Use SHBG to Calculate Free Testosterone:
1. Cost. Both total testosterone and SHBG, at least here in the U.S., are inexpensive lab pulls. You should be able to get both done for about $60 on your own, if your doctor will not do it or your insurance does not cover it, by going through one of these: Inexpensive Testosterone Labs.
2. Delays. From what I have seen total testosterone and SHBG will turn around from the big labs in about 2 days. However, free testosterone will slow down your results often and typically comes back a week later. Check on this with your lab, but has usually been the case.
NOTE: For additional related information, see these pages on Free Testosterone and SHBG. You can read about what is considered hypogonal, or clinically low free T on this page: Free Testosterone Levels.