From the book:
It always struck me as very arbitrary how testosterone deficiency was defined by most practicing physicians. For years most physicians decided to treat or not to treat based on the bottom value of their lab’s total testosterone range. So, for example, consider what this man posted: 
“I got a hold of the lab results from my first testosterone test.
Total Testosterone 194 —— reference range ——-> 241-827ng/dL”
Now this man will have no trouble getting treatment simply because his total testosterone reading of 194 is well below the bottom range value of 241. Basically, the physician trusts the labs definition of what low testosterone is and treats accordingly. However, if the man had a value of 242, most physicians would not treat him, which is unfair in my opinion as some of the labs are using incredibly low levels for hypogonadism.
The labs and physicians are simply not using common sense in my opinion. Why do I say that? Testosterone in the 200’s is below that of the average 80 year old man! Stop and think about it. Why would we definite clinically low testosterone based on the levels of sickly seniors of a very advanced age?!? You can check the numbers out for yourself in my page on Average Male Testosterone Levels by Age.
Evidence as to just how “old school” this is can be shown by the fact that there is one forward thinking lab, LabCorp, that has raised their reference ranges anyway. And LabCorp is the biggest lab in the nation, so, hopefully, others will follow some day. Look at what this poster on the Peak Testosterone Forum wrote:
“It may just be placebo although I doubt it but since having taken the thyroid medication for this little bit it seems I am seeing far less hair on my pillows and noticeably less in my hands after washing my hair. Metabolism seems to have picked up some as well.
Testosterone serum 343 L labCorp range= gold standard (348-1197 ng/dl)
LabCorp changed their entry level range for what is regarded as hypogonadal serum T levels from 240 ng/dl to 348 ng/dl in October 2011. This has helped many men get a more serious consideration from their otherwise unsympathetic GP’s. This new standard has been long overdue for many many years, but finally something has been done about it and LabCorp should be commended. Had this change been made 7 months earlier to when I first had a lab taken at 294 ng/dl in March 2011 my experience regarding testosterone deficiency and the effect it has had on my quality of life may have been very different from what it has been for the past two years.”
I think that one can make a good case that 348 is even a bit low, but at least it is a starting point. And, on a practical level, I can tell you that almost all men writing into the Forum with classic low testosterone symptoms are below this level. We do get quite a few young guys with levels even higher than this struggling however.
Again, it is important to note that there is no guarantee that raising testosterone will correct the underlying issues. However, the point is that it may and, furthermore, many studies show that correcting hypogonadism in the 350-400 ng/dl range will often help alleviate symptoms. And this begs the question: if you can fix a man’s symptoms at 350 ng/dl, then why isn’t 350 ng/dl the threshold for hypogonadism?
Unfortunately, many doctors are not aware of this. They still believe that testosterone is just something you give to the patient to make him feel better about himself but really does very little. Hopefully, the studies below will shatter that myth, and I encourage you to show some of them (in a polite way) to him or her:
1) Arterial Stiffness. Low testosterone greatly increases cardiovascular risk, which can, of course, manifest in many different forms. High blood pressure, low vasoreactivity (arterial responsiveness), arteriosclerosis (arterial plaque) and endothelial dysfunction are some of the primary culprits. As we’ll show below, low testosterone is associated with almost all of these.
Another very dangerous cardiovascular risk factor is arterial stiffness, or hardening of the arteries, and low testosterone has been shown in several studies to be correlated with this condition.  For example, one study of dialysis patients found that men with total testosterone below 235 ng/dl (8 nmol/l) had increased arterial stiffness. Although this study was on a unique subset of the population, it was controlled for a number of important cardiovascular risk factors including BMI (a measure of body fat) and C-Reactive protein (a measure of systemic inflammation). Another example comes from prostate cancer patients, who are often pharmaceutically driven into a hypogonadal treatment state. These men, who are already undergoing cancer treatment, also experience increased arterial stiffness due to the anti-testosterone therapies.
Why is hardening of the arteries so dangerous? The root causes of arterial stiffness are generally loss of flexible connective tissues, decreased vascular muscle tone and plaque buildup. Of course, the latter is famous for also narrowing arteries and raising the potential for higher blood pressure, blockages and stroke. And low testosterone definitely accelerates plaque buildup in the arteries.
Hardened arteries are also dangerous, because they can result in some unexpected phenomenon, such as creating a greater blood pressure spike that can do damage over time. Even worse, stiffened tissues are less able to absorb this pressure spike and thus allow more injury to surrounding tissues, especially in the brain and kidneys.
Does supplemental testosterone help improve arterial stiffness in men with low T? One study tried to answer this question by examining two groups of men, a hypogonadal group with average arterial stiffness of 8.9 and a control group with a lower reading of 6.78. Within 48 hours of testosterone therapy, the low T men were improved to an average arterial stiffness of 8.24 (measured as PWV). The units actually are not important: the point is that testosterone therapy overcame in two days about a third of the arterial stiffness on average that these men had accumulated from years of basement level hormones.
2) Depression. Most men will will not talk about it, but depression is a huge and common issue with men. Again, I see it all the time on the Peak Testosterone Forum. And many guys with low testosterone have experienced firsthand the fact that testosterone profoundly affects the brain. Several studies have verified this and shown that low testosterone can lead to mood issues and depression. The threshold at which this becomes statistically significant appears to be around 350 ng/dl (10.2 nmol/l).  Researchers have also found that testosterone does not just affect how you feel but also how you think. One study observed that low testosterone men experienced memory issues in a manner very similar to that experienced by Alzheimer and dementia patients. 
Free testosterone levels may play even more of a role in this case. One group of scientists rediscovered the 300 ng/dl (10.2 nmol/l) threshold for depression but found that the free testosterone level was 10 ng/dl. This is a little above what is normally considered hypogonadal, about 5 ng/dl, in some of the studies. The odds of depression were found to be between 1.5 and 3.0 times more for the lower quintile guys, depending on whether one used total or free testosterone numbers. The bottom line is that if either free or total testosterone is low enough, get ready for a bumpy emotional and mental ride. 
NOTE: It is extremely critical to get depression under control. Not only does it tend to lower testosterone, it is also a physical stressor than can literally destroy neurons and raise your risk of cardiovascular disease due to elevated cortisol levels.
As anticipated, testosterone therapy benefits men with low testosterone and depression. One study looked at depressed men with testosterone below 350 ng/dl (11.9 ng/dl) and found that when these men were given testosterone therapy (gel), there was substantial improvement.  Of even more weight is the fact that a recent meta-analysis examined seven different studies and found that not only did testosterone therapy improve depression in hypogonadal men, but in men with a broad range of testosterone levels.  One can’t help but wonder how many men out there are struggling with depression who do not realize that low testosterone is playing a role.
In addition, testosterone has been found in several studies to be related to mood.  “Mood” may seem a somewhat subjective word, but, as expected, researchers have studied the subject in considerable psychological detail. For example, one study found that testosterone replacement improved “energy, well/good feelings, and friendliness and decreased negative mood parameters including anger, nervousness, and irritability”.  Furthermore, when studying the use of Androgel, scientists found that “mood parameters improved rapidly and were maintained throughout T treatment.” 
3) Metabolic Syndrome. Prediabetes, when one begins to lose insulin sensitivity, is an absolute epidemic in the U.S. and other modern cultures right now. The current prevalence is about a fourth of the population. Common symptoms, such as visceral (belly) fat, low HDL, high triglycerides and blood pressure, often accompany it in a suite of symptoms called Metabolic Syndrome (Met-S). My guess is that around a half of the guys that show up to my site over the age of 40 probably have Met-S. Researchers have found that Met-S has been tied to an increased risk of developing cardiovascular death and all-cause mortality.  It also increases the risk of erectile dysfunction, since it ages the arteries and accelerates plaque build up.
What does Metabolic Syndrome have to do with low testosterone? As discussed, low testosterone raises insulin levels very significantly and thus directly worsens or initiates Metabolic Syndrome. For example, one study of middle-aged men found that those in the lowest fourth of total testosterone were 1.9-2.5X more likely to develop Metabolic Syndrome over the next 11 years. The threshold for the quartile of total testosterone was 450 ng/dl (15.6 nmol/l).  450 ng/dl would be considered reasonable testosterone by most doctors and yet this study indicates that many men may be getting into trouble near this threshold.
4) Muscle and Fat. A number of studies have shown that testosterone will add muscle and subtract fat. For example, one study on senior men (over age 65) found that fat mass decreased and lean mass increased in the testosterone treated men.  This is truly remarkable considering that the men were put on no special diet or exercise program: their body composition just magically improved for the better with the additional testosterone. It should be noted that these were not large improvements in body composition: it will always boil down to diet and exercise in the end, but testosterone can clearly be a help.
5) Memory. Doubling – actually a little more than doubling – senior males’ testosterone levels led to significant improvements in visual and spacial memory. I can tell you on a practical level that I very often get comments from men on testosterone therapy of the incredible effect that testosterone has on the brain. In fact, some executives now are going on testosterone just to improve their careers! I’m not advising that, of course, but the phenomenon is interesting and you can read about it in this Forum Post.
6) Diabetes. Great improvements in type II (adult onset) diabetes can often be achieved with testosterone therapy in middle-aged and senior men via HRT. Again, I relate the story of one HRT clinic who told me that ALL of their diabetic patients had completely eliminated their need for insulin after testosterone therapy. A couple still had to take Metformin, but, nevertheless, this is a dramatic improvement. Of course, the reason is that increased testosterone led to decreased insulin levels and improved insulin sensitivity in general. Nice!
7) Erectile Dysfunction and Libido. Many studies have shown the positive effect of additional testosterone for men with low T in the area of sexual health. For example, one study on hypogonadal men found that morning erections, total erections and libido were all improved after increased T levels.  The authors wrote: “all had a rapid and dramatic recovery from major depression following testosterone augmentation.” Wow! How many psychological disorders have a 100% success rate?!?
CAUTION: Do not stop any medications or treatments without first talking to your doctor.
8) Venous Leakage. This is an ugly condition that afflicts men, young and old. It very often has a root cause of low testosterone levels. The reason is simple: low testosterone leads to a decay of the internal structures of the penis. This decay does not allow the veins to be closed off during an erection and thus the outflow from the penis is as great as the inflow and erections are either quickly lost or, in the worst case, impossible to even achieve. For more information, see my link on Venous Leakage. And several studies have shown the benefit of testosterone levels to men with venous leakage. One study showed that well over half of men were either partially or fully cured of their venous leakage from testosterone therapy. 
This list could be greatly extended. The point is that eliminating a testosterone deficiency and restoring more youthful levels can significantly or even completely reverse many key symptoms. Should this not be the definition of a true deficiency? And is this something to discuss with your physician?