One of the things I remember about my low testosterone days was the mind-numbing fatigue. Excruciating is the word that comes to mind actually. In the afternoons I was always so tired that I could barely function. One man on the Peak Testosterone Forum with abysmally low testosterone of 144 ng/dl described it as "crushing fatigue."  Cleaning up my diet helped significantly, but HRT (testosterone therapy) made the biggest difference without question. This poster described the same thing:
"Unless it is just placebo or psychological.... I just gotta say- two big symptoms I have had for months has been 'fuzzy' thinking - you know, forgetful and just not as sharp, mentally? Not good especially as a lawyer if I screw up I am liable for it. Also, this terrible fatigue which made me wake up exhausted and then super-tired after doing anything - like ready to call it a day at 2:00 and take a nap. Okay, so when I went to the doctor on Tuesday he gives me a 160mg shot of testosterone sub cutaneous and said a weekly shot might take about 10 weeks before I might feel anything ...... So, I woke up the next day without fatigue or brain fog. Is this in my mind? How could it be? It is very real physically." 
Again, notice that he had painful fatigue that was removed with HRT. Of course, rapid results like this are not everyone's experience, but fatigue is definitely a hallmark symtpom of hypogonadism. One set of researchers wrote that "testosterone replacement therapy or androgen replacement therapy (ART), in hypogonadal men has been well studied. Symptoms associated with male hypogonadism are sexual dysfunction, fatigue, loss of energy, mood depression, change of body composition and osteoporosis."  Still other authors stated that "andropause, a syndrome in aging men, consists of physical, sexual, and psychological symptoms that include weakness, fatigue, reduced muscle and bone mass...sexual dysfunction, depression, anxiety, irritability, insomnia, memory impairment, and reduced cognitive function."  For many of you reading this, those will sound very familiar. I had every one of those symptoms, except for bone loss, for literally decades before finally figuring out I was low testosterone.
As a side note, some hypogonadal men even experience post-orgasm fatigue. Having the big O is not helpful for a man in this situation, but actually takes him out of commission. We've had a number of men describe this situation. Here is one of them:
"Have any of you other low T guys been experiencing fatigue even days after orgasm? Is it a low T symptom? I have a pattern that has been repeating itself. Orgasm once a day for three days. After the last one I am experiencing strong fatigue, even taking on the clothes the day after feels heavy. Standing, sitting, moving, anything feels heavy...If I bend my legs my thigh muscles almost start to shake a little, like they are exhausted, even though I haven t done anything." 
Of course, if testosterone is a root cause of fatigue, one would expect testosterone therapy to be a "fatigue buster." And this is exactly what the research shows. Look at the different forms of HRT that have resulted in improved symptoms:
1. Oral Testosterone Undecanoate. The early forms of oral testosterones tended to be hard on the liver, but oral testosterone undecanoate avoids this issue and a product named Andriol has been around since the 80's according to my understand. Researchers not that "using a one-tailed test of significance, 'fatigue' was lower and 'vigour' higher on TU." 
2. Testosterone Enanthate (Supraphysiological). Going supraphysiological, i.e. above normal youthful levels, is not a good idea for many reasons. However, some guys do it for a short while when they begin HRT and for reasons unknown researchers examined this with testosterone enanthate, which is very similar to testosterone cypionate. The found that "significant reductions in negative mood (tension, anger, and fatigue) followed by an increase in vigor were found in response to T treatment in the hypogonadal group."  In other words, this proved to be a rapid fatigue blaster as one would expect. However, it also increases the risk of side effects...
3. Testosterone Undecanoate Injection (Long Term). There is documented evidence that the benefits of HRT have a long term lasting effect as well against fatigue, i.e. this isn't just a short term effect that goes away. One study on Klinefelter patients showed that even after 3.6 years patients "showed better mood, less irritability, more energy and drive, less tiredness, more endurance and strength, less need for sleep, better concentration ability and better relations with others during testosterone treatment...Treatment was characterized mainly by more endurance and strength, less fatigue." 
CAUTION: Some men on testosterone therapy will actually experienced increased fatigue. This may be from unmanaged estradiol, but the cause is not always known. And some men will have the opposite experience: they will feel SO good that they "go crazy" in the gym and end up with strained ligaments and tendons!
3) International Journal of Impotence Research, 2007, 19:411 417, "Transdermal testosterone gel increases serum testosterone levels in hypogonadal men in Taiwan with improvements in sexual function"
4) Pharmacotherapy: The Journal of Human Pharmacology and Drug Therapy, Aug 1999, 19(8):951 956, "Testosterone and Andropause: The Feasibility of Testosterone Replacement Therapy in Elderly Men"
6) Clinical Endocrinology Volume 14, Issue 1, pages 49 61, Jan 1981, "ANDROGEN REPLACEMENT WITH ORAL TESTOSTERONE UNDECANOATE IN HYPOGONADAL MEN: A DOUBLE BLIND CONTROLLED STUDY"
7) Physiology & Behavior, Apr 2002 75(4):557 566, "Exogenous testosterone, aggression, and mood in eugonadal and hypogonadal men"
8) Clinical Genetics, Apr 1988, 33(4):262 269, April 1988, "Follow-up of 30 Klinefelter males treated with testosterone"