Overtraining can make you sick, lower your erectile strenght and libido, lead to a collapse of adrenal hormones and even lower testosterone, something I discuss in my page on Overtraining and Testosterone. And there have even been cases, if OTS (Overtraining Syndrome) goes on long enough, that is has quite literally destroyed the individual's health. You might as well sign up as a prisoner of war in some remote corner of the planet - it's essentially "anorexia for men."
As men we are trained to ignore pain, much less the more subtle issues that arise from something like overtraining. And, unfortunately, some of the best measures of overtraining are fairly subtle and subjective. These include "poor performance in competition, inability to maintain training loads, persistent fatigue, frequent illness, disturbed sleep and alterations in mood state."  Unfortunately, if a guy can ignore major pain and discomfort, he will probably also easily ignore these kind of "soft" tests for overtraining as well.
Plus, a problem that I have seen a number of times on The Peak Testosterone Forum is that guys who are overtraining can become like addicts. They will even sometimes self-medicate with caffeine or energy drinks. Poor sleep may come on slowly to where they almost forget what a good night's sleep is like. I have even known a marathoner who basically walked around with a cold all the time and said it was "allergies."
So on this page I want to laser in some of the more objective measures of overtraining in the hopes that perhaps the hard numbers will get some guys to think a bit that maybe, just maybe they are overdoing it a little and that they could be compromising their long term health by doing so.
Belwo are my current Four Best Signs of Overtraining via standard lab results for guys who are ignoring everything else. If you know someone who you are pretty sure is overtraining, I am hoping you can convince to use these numbers to make your case. I also want to mention that there are actually other excellent measures that one can use (adrenal hormones, testosterone/cortisol ratio, catecholamines, waking pulse, etc.), but these, unfortunately, require before and after data. And most guys overtraining do not have an athletic trainer forcing them to continually monitor these kind of numbers.
1. Hematocrit and Hemoglobin. Various markers of iron status have been found by many experts to be good markers for overtraining syndrome. One of our posters explained why:
"There is lots of evidence here to suggest over-training. For example, a paradoxically and persistently low hemoglobin with no other explanation is consistent with a phenomenon called "march hemoglobinuria and hemolysis" due to excessive trauma to the feet and red blood cells. Trail marathons and long runs that last 3-4 hours are also excessive for most people. The author of the post also mentions that he notices a "reduction over time in my libido levels and muscle recovery after long runs or lifting." 
What should you look for? First of all, let's look at the normal ranges for the latter two in men:
Hemoglobin: 14-18 g/dl
The bottom line is that if you are on the lower end of these ranges, you may want to consider the possibility of overtraining. This is especially true for men on testosterone therapy, whose hemoglobin and hematocrit will rise (assuming their TRT program is actually working and boosting their T).
Consider this statement from a research review that explained common signs of overtraining:
"Low levels of haemoglobin that remain within the normal range and iron deficiency are commonly recorded conditions among some groups of professional athletes, both female and male. These types of deficiency may result from an unbalanced diet, gastrointestinal (GI) bleeding during and after long distance running and, possibly, chronic low-grade systemic inflammation." 
In other words, just because you are in range does not mean you are "out of the woods." Again, being in the low end of the range is often a sign - more on that below:
2. Ferritin. Related to iron status above is ferritin, which is a measure of the body's iron stores. Think of hematocrit and hemoglobin as a snapshot of current plasma iron levels, but ferritin a measure of the actual iron that gets stored in the body. Ferritin is a great marker, because many athletes have been known to get low ferritin from overtraining.  Does this occur because of the low hematocrit and hemoglobin mentioned above? This probably plays a role, but the actual driving force behind the low ferritin is actually much more insidious:
a) Excessive exercise leads to high levels of inflammatory cytokines. 
b) These cytokines cause increased levels of hepcidin, a hormone that controls iron.
c) Rising levels of hepcidin decrease iron absorption in the gut.
The bottom line is that if you have ferritin levels below about 25 ng/ml, you may be overtraining.
3. Low IgA. Because of the seemingly high incidence of URTI [UpperRespiratory Tract Infections] among athletes, much attention has focused on the mucosal immune system response to intense exercise training, using salivary IgA concentration as a marker. Low resting salivary IgA concentration has been reported in some elite athletes. Salivary IgA levels decline during prolonged periods of intense exercise training and IgA concentration is lower in overtrained compared with well-trained swimmers. Low salivary IgA concentration is predictive of the subsequent appearance of symptoms of URTI over the short and long term.""
4. A/G (Albumin/Globulin Ratio). Many men know overtraining can literally make you sick and increase the risk of upper respiratory infections. However, some men just don't get sick very easily. So what is an early warning sign, ideally a lab test, that might show issues in this area? One of them is a high A/G ratio, which basically shows immunosuppression, i.e. an immune system that is functioning suboptimally. It is important to talk to your doctor about this, because it can in rare cases be a sign of something serious, such as leukemia.
However, a high A/G ratio can also tie into overtraining. It turns out that overtraining is known for suppressing the immune system - it's kind of "Cancer Lite" if you will on your immune system, something I alluded to in #3 above. One study stated it this way: "A tentative trend may be discerned whereby light to moderate exercise may increase immune responsiveness but high-level competition sport, especially if it involves extensive endurance training, may lead to a degree of immunosuppression."  Thus, overtraining can suppress immunoglobulins (IgA) and can lead to an elevated A/G ratio as well. Check out these three interesting studies done in one paper as an example:
"Study 1: Recreational joggers ran on a treadmill for 40 min at 55% and 75% VO2peak and competitive distance runners ran for 90 min at the same intensites. In both groups, IgA secretion rate did not change significantly after exercise at either intensity.
Study 2: Competitive runners ran on a treadmill for 90 min at 75% VO2peak on 3 consecutive days. IgA secretion rate decreased 20 to 50% after exercise (p < .001). Post-exercise IgA secretion rates were significantly lower (p < .05) on days 2 and 3 compared with day 1.
Study 3: Elite swimmers were followed over a 6 month season, with IgA concentration measured at 5 times. Throughout the season, IgA concentration was significantly (p < .05) lower in stale compared with well-trained swimmers." 
The latter comment is particularly interesitng, because swimmers that were not improving, perhaps the key marker of overtraining, also tied with low IgA concentrations. This indicates that IgA is very valid sign of overtraining.
NOTE: I hope you noticed this common theme in the research: light to moderatre exercise is incredibly good for you. More intense and long gets questionable very quickly.
High BUN and Creatinine. These numbers are often high in endurance athletes (and bodybuilders). The reason? Both of these tend to be high from to the breakdown of muscle that occurs in these sports. Endurance athletes also tend to be dehydrated, which further elevates these values.
Recently, I was looking at the lab work of a marathoner that had some of the classic signs of overtraing, such as irritability and reactive hypoglycemia in the middle of the night. (It was effecting his erections as well.) Here were his results (from LabCorp):
BUN 20 mg/dl Range: 6 - 24 mg/dl
Creatinine 0.9 mg/dl Range: 0.76 - 1.297 mg/dl
Should he be concerned? The rule of thumb that I have heard is that, if you are near the top of the range for an exteneded period of time, you could be putting a lot of wear and tear on your kidneys. The exact value where that occurs probably anyone's guess.
One thing to notice is that his BUN/creatinine ratio is high (22) which probably indicates dehydration, again a comon problem in athletes. (It can also indicate something more serious, so run it by your doc to play is safe.)
NOTE: Taking creatine can raise BUN a little as well, but long term studies have shown no kidney issues in those with no kidney disease.
1) Br Med Bull. 1992 Jul;48(3):518-33, "Sport and the overtraining syndrome: immunological aspects"
2) Int J Sports Med, 1994 Oct, 15 Suppl 3:S179-83, "Mucosal (secretory) immune system responses to exercise of varying intensity and during overtraining"
3) Immunology and Cell Biology, 2000, 78:502 509, Special Feature for the Olympics: Effects of Excercise on the Immune System, "Overtraining effects on immunity and performance in athletes"
5) J Sports Sci Med, 2013 Jun, 12(2):249 258, "Do High Blood Hepcidin Concentrations Contribute to Low Ferritin Levels in Young Tennis Players at the End of Tournament Season?"
6) Med Sci Sports Exerc, 2000 Feb, 32(2):317-31, "Cytokine hypothesis of overtraining: a physiological adaptation to excessive stress?"