Free Testosterone Levels
STEP 2B. Consider your Free Testosterone Levels if Needed:
There are two reasons that I know of to pull your free testosterone:
a) If you have medium or low medium total testosterone, it is possible to have proportionately lower free testosterone if you have high SHBG. The low free testosterone, from what I have seen produces just as many symptoms as low total testosterone, and this is because it is the free testosterone that really acts on the tissues anyway.
b) Many studies use both total and free testosterone as independent markers of hypogonadism.
Also, another reason to understand free testosterone levels deals with a question that I get quite often on the Peak Testosterone Forum:
"My doctor says my free testosterone levels are fine, but I still feel lousy. Are my levels really okay?"
That's a tough question to answer and the reason is simply that there is such a wide variety of answers out there. I'm going to cover some of the free testosterone hypogonadal ranges that I have seen out there and you'll see that this is not easy to get your arms around. And it is, by the way, one the reason that I use total testosterone in my discussions with men about the subject. Doctors, labs and researchers seem very divided on free testosterone, which is not nearly as much of the case as with total. For example, the unspoken rule of thumb that I now see out there with urologists and PCPs is that, while upper 200's (in ng/dl) for total testosterone is considered hypogonadal, a man can get HRT if he is the 300's. So the variation between hypogonadal and treatable is maybe 30%.
Again, this is not the case with free T and let me give you some examples below. I do want to say one thing about the units involved first. All my numbers below are in ng/dl and often free testosterone is given in pg/ml. The conversion is simple: you simply take the free T number in ng/dl and multiply it by 10 to get the equivalent in pg/ml.
1. 1.5 ng/dl (Journal of Urology). This is the lowest threshold that I have ever seen for defining hypogonadism:
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"Low levels of TT and FT were defined as less than 300 and 1.5 ng/dl, respectively. Markedly low levels of TT were defined as less than 200 ng/dl." 
We'll go into this a bit later, but 1.5 ng/dl is VERY low free testosterone.
3.5 ng/dl (Quest Diagnostics). One of the biggest labs in the United States is Quest and they list, for their top tier LCMSMS test a range of 35-155 pg/ml for an adult male aged 18 to 69.  As mentioned above, this translates to 3.5 ng/dl.
4.0 ng/dl (LabCorp). The biggest lab in the U.S. is LabCorp and they weigh in with a hypogonadal free testosterone threshold of 4.0 for an adult male. 
4. 6.5 ng/dl (Cleveland Clinic Journal). This journal weighs in with a significantly different number:
"Free testosterone levels can be diagnostic when total testosterone levels do not
correspond with clinical presentation. However, the clinical utility of free testosterone is
difficult to assess due to the variability among laboratory assays and a lack of consensus on threshold
parameters. A threshold free testosterone level of more than 225 pmol/L (65 pg/mL) is generally considered normal."
5. 10 ng/dl (For Depression).
Free testosterone levels may play even more of a role in the case of depression. One group of scientists found that the lowest quintiles of
total (300 ng/dl (10.2 nmol/l)) and free testosterone (10 ng/dl) were the threshold for greatly increased risks of depression, 1.5 and 3.0 times,
respectively. The bottom line is that if either free or total testosterone is low enough, get ready for a bumpy emotional and mental
6. 13.5 ng/dl (For Depression #2). Yet another study on men being treated with antidepressants
found that levels of 13.5 ng/dl may be associated with sexual dysfunction.
 The authors wrote that "free testosterone levels were found to be subnormal in 15 of 20 patients. No other consistent laboratory value nor physical examination finding could account for this observation...The male ASECS mean score was 20 with a mean free Testosterone of 13.5 pg/ml."
Another Way to Think of Your Free Testosterone Levels
As you can see from the above discussion, the problem with free testosterone is that there is no consensus as to what is hypogonadal, or clinically low testosterone. In fact, not only is there no consensus, there is a huge range in the values that are used by researchers and labs. This makes it all but useless for your doctor and my guess is that that is why they seldom use it. (Another reason is that bioavailable testosterone is conceptually a much better number but is grossly understudied.)
So we should throw out free testosterone? Well, I certainly don't think so and I will give you an example where
I believe it to be incredibly useful and, furthermore, a practical discussion point for some men with their physician. In
fact, let me go on record by saying that I think a fairly reasonable hypogonadal number is probably around 8 ng/dl. The
reason is simple. As you'll see in my book, the risk of all the medical conditions (diabetes, venous leakage, depression,
prediabetes, osteoporosis, hardening of the arteries, etc.) all occur in the total testosterone range of 300-400 ng/dl range. (For
you it could be a little higher or lower: the studies just deal in averages.) This is something I document in my
book Low Testosterone by the Numbers.
To calculate how that 300-400 ng/dl of total testosterone translates to free testosterone, one just has to remember that the average free testosterone percentage for a healthy male (not on HRT) is about 2.0%. Thus, 300-400 ng/dl total testosterone is roughly equivalent to 6-8 ng/dl. I would choose 8 ng/dl to play it safe, i.e. give the benefit of the doubt. Again, symptoms are just as important as the number and we are all different, so your hypogonadal free testosterone number could be a little higher or a little lower.
PRACTICAL APPLICATION: As mentioned above, I have seen a few guys on
Peak Testosterone Forum who have had classic low testosterone symptoms with total testosterone around 500 ng/dl. Normally, men do not have too many issues in the 500's, but, upon further questioning, we usually discover that their free testosterone percentage was quite low - near 1.5%. And, if you look at the lab ranges for free testosterone percentage, you'll see that it generally says 1.5-2.5%. Therefore, men with a low free testosterone are simply at the lower end of this range.
But let's do the math. Again, let's take a guy with total testosterone of 500 ng/dl and a free testosterone percentage of 1.5%: he will have a
free testosterone level of .015*500 = 7.5 ng/dl. And, again, notice that this 7.5 ng/dl corresponds very closely to the 8 ng/dl that I calculated above.
And this is an argument that many doctors will listen to. You can explain, assuming you have a low free T percentage, how your total testosterone in the low 500's really corresponds to total testosterone in the 300's. And he (or she) may just listen.
THE EASY AND MORE ACCURATE METHOD: In my opinion, free testosterone is often an unreliable test. What I see the more savvy (by my definition) doctors doing is pulling SHBG and total testosterone and plugging that into and online free testosterone calculator. This is what most often produces meaningful results that make sense. You can play around with the numbers yourself. For example, I ran the numbers with an SHBG of 50 and total testosterone of 500 and the calculator spit out a free T of 1.6% and 8.02 ng/dl. So 50 and above is about when you'll start getting these lower free testosterone percentages. See my link on
High SHBG for more information.
1) British Journal of Cancer, 2000, 83(1):95-97, “Hormones and diet: low insulin-like growth factor-1 but normal bioavailable androgens in vegan men”
2) The Journal of Urology, Sep 2003, 170(3):795-798, "The Value Of Pituitary Magnetic Resonance Imaging In Men With Hypogonadism"
http://www.questdiagnostics.com/testcenter/testguide.action?dc=TS_Testosterone_LCMSMS , Test Summary, Testosterone, LC/MS/MS, Clinical Use, Diagnose and monitor disorders associated with testosterone abnormalities (Table 1).
Testosterone, Free and Weakly Bound, Free and Albumin-Bound Testosterone, Test Number: 143255 CPT Code: 84402; 84403
5) Cleveland Clinic Journal of Medicine, November 2012, 79:11:797-806, "Androgen deficiency in older men: Indications, advantages, and pitfalls of testosterone replacement therapy"
ARCH GEN PSYCHIATRY, MAR 2008, 65(3):283-289, "Low Free Testosterone Concentration as a Potentially Treatable Cause of Depressive Symptoms in Older Men"]
http://www.priory.com/psych/sexdys.htm, "Antidepressant-Induced Sexual Dysfunction Associated With Low Serum Free Testosterone"