If you’ve been struggling to gain muscle mass – and a lot of guys do – this subject should interest you. Let me tell you that the results can be spectaculr according to what I hear on The Peak Testosterone Forum. Look at what these men wrote for example:
“I have always stayed in shape but noticed I had lost strength and muscle mass, and not being able to sustain an erection very well. I am 63. I feel better now [after HRT] than I have in 30 years, have gained 8 pounds of muscle and lost 2 inches in my waist. I am almost as strong now as when I was in my mid 30’s. Sex, yes, 3-4 times per wk. Would be more but my wife said this is enough.” 
“In my situation I believe I have basically borderline low testosterone most of my adult life and even as a teen I was surely low for the norms of my age group. I have struggled with all the normal symptoms, Hard to gain muscle, hard to lose fat, very passive personality, low motivation to follow through and a general apathy towards most things being neither depressed or excited.” 
“Hello, I’m a 27 year old male, weigh 168lbs just a few questions i went to the doctor a while back as i dont get morning erections as frequently as i used to and have a hard time building muscle in the gym, always have. i started lifting weights properly about 1 and a half years ago (i was about 144lbs then and could bench press 40kg for a few reps) now my 1 rep max for bench press is 100kg and can bench press my body weight about 10 times, so i have clearly made gains. i asked for a testosterone test, i had a morning test but im in england and they didn’t seem to do all that much, i got my blood test but the ONLY thing they tested for was total testosterone which in my case was 14.6 which i believe is about 430 ng/dl. ” 
Of course, these are all anecdotal and very subjective stories. Nevertheless, this is a common theme that I see on the forum. However, the most important question is: “What does the research say?” Below I will look at three studies that show how increasing testosterone increases muscle, plain and simple (and then one controversial study that says the opposite):
1. Healthy Young Men. One of the most interesting studies was actually on young men where participants were between 18 and 35 years old. The researchers wanted tight control of their testosterone levels and thus suppressed the natural (endogenous) testosterone of the men in the study with an anti-androgen and then grouped them in five different groups based on dosage levels of testosterone therapy (using testosterone enanthate).
2. Seniors. What about us older guys? Most men over 65 have lost around half or a little more of their youthful, peak testosterone levels. One study looked at giving men in this age category testosterone patches and then monitoring their body composition. The changes were remarkable, considering that no other changes were implemented, i.e. no strength or resistance training. Many positive changes occurred, but the one I wish to focus is an increase in “lean mass”. 
What is “lean mass”? Lean mass is essentially muscle + bone. Of course, your bones do not change significantly and, therefore, an increase in lean mass generally means a nice increase in muscle. (Now, technically, it includes water weight, but this should not be an issue in this study.) Again, by all indications, the participants gained a nice amount of muscle even though well into their senior years and doing no weight lifting.
3. Hypogonadal Males. Similar to study #2, this study looked at men with clinically low testosterone, i.e. hypogonadism. Of course, some of the men in #2 would be hypogonadal, just from aging, but here all men in the group were required to have very low testosterone. Of course, the results were what we would expect: increases in fat free mass. The authors concluded that “testosterone replacement in hypogonadal men enhanced skeletal muscle mass by stimulating the muscle protein synthesis rate.”
Did you know you can inexpensively do your own testing for most hormones? The industry leader is Discounted Labs..
REBUTTAL: I should mention that there are a few opposing voices. For example, one 2012 study showed that testosterone is completely unrelated to muscle gains in young men.  Interestingly enough, the study showed that cortisol was much more correlated to muscle growth than testosterone. The same author did a similar paper which showed that women, who have much lower testosterone levels, had equal muscle protein synthesis to men and, therefore, that testosterone has little to do with muscle growth.  The author was very emphastic and stated that “while testosterone is definitely anabolic and promotes muscle growth in men and women at high doses, such as those used during steroid abuse, our findings show that naturally occurring levels of testosterone do not influence the rate of muscle protein synthesis.” Time will tell I guess…
Now this brings up an important point: even at lower levels of testosterone, you can still put on muscle. It is dose dependent after all. A little testosterone means a little growth and a lot of testosterone means a lot of growth. However, we will cover below how low T guys can still do quite well.
CAUTION: I am not advising anyone to go on HRT (Hormone Replacement Therapy) . You have to do your own research and discuss with your physician depending on your testosterone levels, preexisting medical conditions and other factors such as desire for fertility, etc..
I am an example of this. I was in the 300’s (ng/dl) for years – perhaps decades – and put on at least 20 pounds of muscle since college. I always thought to myself, “Well, it doesn’t come on as easily as when I was 20, eh?” But, nevertheless, I steadily gained both mass and strength even when my testosterone was very low.
And we have had several similar stories on the forum. Consider what this guy wrote:
“My Total T has ranged from 350-390 the last two years. Morning wood is a thing of the past, and I rarely am interested in sex. I am very healthy and go to the gym a lot and have quite a bit of muscle..” 
Of course, notice that this guy also has quite a bit of muscle even though his testosterone has only been in the upper 300’s. You have to realize that even if your testosterone is 300, it is likely about 10X that of your wife and girlfriend. So you’re still very much male and capable of gains. Now you may be a miserable male struggling with erectile dysfunction, low libido and Venous Leakage – been there! – but you can still slowly add muscle.
2) Bhasin S, Woodhouse L, Casaburi R, Singh AB, Bhasin D, Berman N, Chen X, Yarasheski KE, Magliano L, Dzekov C, Dzekov J, Bross R, Phillips J, Sinha-Hikim I, Shen R, Storer TW. “Testosterone dose-response relationships in healthy young men. Am J Physiol Endocrinol Metab. 2001 Dec;281(6):E1172-81
3) The Journal of Clinical Endocrinology & Metabolism, Aug 1 1999, 84(8):2647-2653, “Effect of Testosterone Treatment on Body Composition and Muscle Strength in Men Over 65 Years of Age”
4) https://www.mcmaster.ca/opr/html/opr/media/main/NewsReleases/ Researchdebunksbody buildingmythgrowthpromotinghormonesdontpromotegrowthorstrength.htm
5) European Journal of Applied Physiology, Jul 2012, 112(7):2693-2702, “Associations of exercise-induced hormone profiles and gains in strength and hypertrophy in a large cohort after weight training”
6) Journal of Applied Physiology, Jun 1 2012, 112(11):1805-1813, “Sex-based comparisons of myofibrillar protein synthesis after resistance exercise in the fed state”
7) The Journal of Clinical Endocrinology & Metabolism, Oct 1 1996, 81(10):3469-3475, “Effects of testosterone replacement on muscle mass and muscle protein synthesis in hypogonadal men–a clinical research center study.”