The benefits of testosterone therapy (HRT / TRT) were absolutely incredible for me. I was low testosterone all of my adult life and testosterone gave me morning erections for the first time; helped with some loss in erectile function and libido; and, best of all, got rid of life long dysthymia (mild depression) that I had experienced for decades. In my case testosterone therapy produced dramatic results and really gave me a quality of life that I simply had not experienced as an adult. It was beautiful.
And it is really very strange, if you stop to think about it, that you can increase one hormone along the pathway from cholesterol breaking down into such major hormones as DHT, estradiol and cortisol, and produce such a dramatic effect on so many things. If you increase DHT or DHEA or pregnenolone, you just don't get those kind of profound and widespread benefits generally. (You can read about my experience if you interested in My Personal Health Story.)
Below is STEP 9 in my Testosterone Evaluation Program, which includes Over 15 Very Common, Research-Backed Benefits of Testosterone:
1. Venous Leakage Reversal (Partial or Complete) and/or Erectile Dysfunction. You can have pristine penile arteries that pump out all the nitric oxide in the word, but, if you have venous leakage, you're not going to be able to get or maintain an erection. Basically, venous leakage is when your penis cannot trap the blood that flows into it, i.e. as much blood leaks OUT of the penis as enters in. A common symptom associated with venous leak is a rapidly disappearing erection. This can cause major problems in the bedroom as some men will lose their erection just walking over to get a condom or walking over to make sure a door is locked!
The most common reason for this is a surprising one: low testosterone. It turns out that when a man's testosterone level gets low enough, the nerves and connective tissue inside the penis begin to atrophy. Under this scenario the penis just cannot lock off the veins that carry the blood out of the penis and so inflow and outflow become very nearly equal. In addition, low testosterone can lead to a loss of morning erections and less sexual intercourse, which decrease oxygenation of the penis.
The good news is that testosterone given to a hypogonadal male has been shown in several studies to reverse, partially or totally, venous leakage. For details on the research, see my page on Venous Leakage.
HRT can also significantly improve erectile dysfunction itself. Part of this for some men may be its improvement of venous leakage. But testosterone, as I'll discuss below, also raises dopamine, increases endothelial nitric oxide, decreases hardening of the arteries and inflammation and many other parameters that will likely greatly improve erectile strength. See my page on Testosterone and Erectile Dysfunction for more information.
CAUTION: Now, before I go on, I want to mention that not every hypogonadal man gets these kind of results. On the Peak Testosterone Forum there are just as many guys that little or no benefit and some even have side effects. I cover the dark side of HRT (TRT) in my page on The Risks of Testosterone, and I urge everyone to scan through it. Sometimes HRT-related issues can be explained by a poorly done or poorly monitored protocol. However, that is certainly not always the case and so you have to be realistic in your expectations. Furthermore, the root cause of your problems is not always low testosterone. Of course, it is important to find a knowledgeable and experienced physician and run everything by him or her first.
2. Nitric Oxide. A less well-known property of testosterone is that it activates endothelial nitric oxide. This is a powerful anti-inflammatory and arterial protector: Testosterone and Nitric Oxide.
3. Libido. Many women find it hard to believe that a man could ever have zero sexual desire, but low T will do just that. (So will many other things such as high prolactin, hypothyroidism, etc.) In fact, a couple of studies show that libido rises with testosterone in a dose dependent manner. To check out the research, see my page on Testosterone and Libido.
4. Restored Morning Erections. It is no secret that testosterone therapy can restore morning erections, especially if the testosterone levels are high enough. As I mentioned, I did not have morning erections until I went on HRT. More accurately, I did not get morning erections until my testosterone levels were consistently above about 600 or 700. We have had other posters make the same claim, i.e. that morning erections were returned after a long period of absence due to low T. 
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All of this has been verified by the research. A number of studies have shown that nocturnal erections are increased and improved after testosterone replacement. However, it is important to note that recovery from lack of morning erections can take time. One study found that morning (and spontaneous) erections improved steadily over the course of 12 months.  This is likely because the body must literally rebuild tissues and receptors within the penis.
5. Decreased Anxiety. Many low testosterone men struggle with anxiety. What is not so well-known is that as one testosterone drops, anxiety can go through the roof. When I went on injections, that was one of the first things I noticed: I finally felt peaceful and not so stressed out. My boss actually made a comment to me and asked what had changed! See my page on Testosterone and Anxiety for more information.
The reasons for this are probably numerous, but one of the most intriguing ones is that testosterone has been found to limit the cortisol response from CRH at the adrenal. In other words, testosterone limits your body's stress response. See my link on Testosterone and the Adrenals for more information.
Whatever it is, look at the example below as to just how powerful this change can be for a man that increases his T from basement levels:
"I'm also much "calmer" than before and mind seems better able to focus. You would think raising T levels would make you prone to anger and violence? But it has had the opposite effect on me." 
6. Fatigue. When I was low testosterone, I was excruciatingly fatigued and this is a common complaint on the forum. Again, HRT dramatically helped with this and this is something I simply do not see in the studies. It makes sense, though, as the studies tend to look at the more quantifiable medical conditions.
7. Desire to Workout. One of the most common comments about HRT is actually that men frequently feel like exercising and hitting the gym again. Hard workouts become the norm instead of the exception. In fact, my HRT clinic warned that men on HRT who complain about joints usually do so because now they are working out hard again and perhaps pushing themselves a little hard, as opposed to low estradiol. And I believe it based on some the comments that I see quite frequently on the forum such as these:
"At 44 yrs old this is the best I've ever looked or felt considering the fact that I was preparing to meet my maker 3 yrs ago! I honestly go to the gym now 5 days a week 3 hrs. a day and actually working out the whole time, and I look forward to it since I have the energy now. I'm now at 225 lbs with about 10% body fat and I'm seeing my body change in ways that I've never seen before. I have more energy now than when I was 17. Plus my wife is really liking it because she says that I'm acting 17 again when I comes down to our bedroom time 'which is a lot more now day's' If you know what I'm talking about!" 
"This weekend I felt like I had to lift because I just took a T shot a day before and could not take a day time nap because I felt like I could not sit still. I went to the gym and ran a mile under 7 minutes, did hardcore ab workout with ab straps doing leg raises and knee raises, and lifted back intensely. Afterwards, I felt a weight off of my impatient feeling like a drug almost. I felt incredible. I love the fact that workouts for me does lifting of my spirit and takes a lot of stress and weight off of me. I do not mean a foggy depressed weight in my head but more so an urge to use my testosterone as a man. I told my wife at that point I had to go to the gym, not to get mad at me, this was an urge that had to be fulfilled. She understood from what she said she has read with treatments. I came back home normal and balanced. It is so nice to have this feeling that I can utilize in such a great way and come home even better than woken up." 
8. Weight Loss. What is not so well known about testosterone is that quite a few studies show that you can lose weight simply by going on testosterone therapy. You don't have to increase exercise. You don't have to watch or cut back on calories. All you have to do is boost your testosterone (assuming you are low T of course). Now no study shows that testosterone is going to melt 50 pounds off of you. However, the studies do seem to show that HRT can help you lose a little weight, something I outline in my page on Testosterone and Weight Loss.
9. Lowered Depression. As far as I remember, I never had a morning erection in my adult life, so there is strong chance in my opinion that I was low testosterone my entire adult life. And I struggled post-puberty with another common symptoms of low testosterone: depression. My depression was generally mild and was like "having the blues" or a feeling of melancholy almost continually. The remarkable thing for me was, when I went on testosterone cypionate, the depression disappeared literally overnight. I have never felt depression a day since.
Now not everyone has this transforming kind of a change with HRT. And it is important to realize that cypionate probably doubled my testosterone levels. Nevertheless, many men report significant improvement in depression on The Peak Testosterone Forum and this is supported often in the research as well. For more information, see my page on Testosterone and Depression. [Do not stop any medications without consulting with your doctor first.) In my opinion, if a man is depressed, one of the first things he should do is pull his testosterone. It's a $25 test and can be done without a doctor's orders: Inexpensive Testosterone Labs.
10. Sleep. As far as I know, the researchers have not really examined or quantified just how much testosterone affects sleep. Typically, they are looking at the more "sexy" parameters. However, I can tell you that I and several of the guys on the forum have had horrible sleep when they were hypogonadal. I know that I used to very commonly wake up with the sheets soaked, all of which went away once I got my testosterone levels up sufficiently. Look at what some of the other men on the forum have reported:
"I can tell you I noticed a huge improvement in my sleep quality after going on Tcyp. I sleep harder for sure and I don't wake up at every little noise anymore, not even my wife's snoring!" 
"I never slept good in my life until I got on testosterone. I sleep 7-8 hours a night usually now unless I'm really tired and then I meet sleep 11. Before that, I would get depressed and sleep 12 hours a day and still feel tired."
11. Insulin, Prediabetes and Diabetes. This is the least sexy of the benefits of testosterone, but it is probably the most important. Basically, as testosterone rises, insulin levels and resistance fall. Because of this, a hypogonadal man that goes on a good TRT protocol can actually reverse his adult onset diabetes. I discuss this in considerable detail in my pages on How to Reverse Diabetes and Testosterone and Diabetes.
12. Inflammation. Okay, I lied to you in step 10: testosterone's primary benefit may be it's powerful anti-inflammatory action, especially in the arteries. I cover this in my page on Testosterone and Inflammation.
13. Marriage and Relationships. Relationships take a lot of work, especially during the more difficult times of life. Having no libido, no energy, mental fog, fatigue and mild erectile dysfunction is certainly not going to help! And we have had many men on the forum that have felt strongly that low testosterone was jeopardizing their relationship or marriage. For more information, read my page on Testosterone, Marriage and Relationships.
14. Career Building. Guess what the hottest drug on Wall Street is according to a recent ABC article? Yep, it starts with a 'T'.  For example, check out this commentary from the article:
"He said about 90 percent of his patients are guys who work in finance traders, CEOs, upper-level management. And it's not just the little guys or executives from small companies. He s got patients from some of the biggest names in finance, including Goldman Sachs, Citigroup, Bank of America , American Express , and Morgan Stanley. The patient list at Cenegenics also reads like a Who s Who of Wall Street: Goldman Sachs, JPMorgan, Deutsche Bank, Bank of America, traders from the New York Stock Exchange and Nasdaq and a lot of hedge funds. Many complain that they used to love going to work but now they're losing their edge. Their head s not in the game as much."
So it's no secret on Wall Street that testosterone can help the brain and there have been some articles and commentary lately that many executives are now on HRT primarily for the mental, career-building benefits.
15. Big Bedroom Boost. Middle-aged and senior guys will usually get the biggest bang - no pun intended - for their buck from PDE5 inhibitors, simply because usually their arteries are in much worse shape than their hormones. However, that is not always the case and many men - especially young guys - get incredible bedroom benefits from HRT. If you don't believe me, check out the descriptions below. And try not to get jealous while you're at it!
"Finished the night with sex that ABSOLUTELY felt more like it did when I was younger. My whole body was alive, and the orgasm was incredibly strong. It just usually doesn't work that way anymore." 
"I make love to my wife on an average 6-8 times a week. I am taking advantage of this feeling because I never had it before and I am 34 years old. I love it. I am finally at the point where my wife has had enough in the bedroom and I am asking for more. This is the first in my life I ever felt so good and confident." 
NOTE: If you read the article, it does have some information such as claiming that testosterone is expensive. Injections and compounding are actually very reasonable and a number of men on the forum do not have insurance and use these options.
16. Anemia. Struggling with fatigue? Feel achy and drained? I sure did when I was hypogonadal. HRT seemed to slowly melt all that away. One reason that many men do not realize is that this can be partially due to anemia. In fact, it is quite common for low testosterone men to end up anemic, which of course can lead to fatigue and many other unpleasant symptoms. The reason for this is that testosterone stimulates a kidney protein that triggers red blood cell production. This is why men that go on HRT have to get their RBCs, hematocrit and hemoglobin regularly monitored. The good news is that several studies have shown that men who are anemic due to a testosterone deficiency can correct the problem through replacement therapy. 
17. Spatial and Verbal Memory and Function. Several studies show that low testosterone men do worse in verbal and spatial mental performance. The good news? Other studies show that men significantly improve in these areas after replacement therapy. 
18. Super Brain Power. As mentioned above, the most immediate and powerful benefits of going on HRT is often in the brain. Now that isn't always true, but many men have reported that the first night they can strongly feel the effects of testosterone right between the ears. I have always described the experience of my first night on testosterone cypionate injections as "fireworks going off in my head." I know it sounds like I am exaggerating but look at what other men have written on the forum:
"On another note I injected Monday for the first time. Yesterday I could tell a difference just a bit. By 1 hour into my shift today I felt like the energizer bunny had hooked up to me for charge up:-)) I was completely shocked at how I felt today. I was told it will get better to in days and with the injections to come over the next weeks. I thought WOW. I felt ready to go to the gym after work to." 
"Well, I have been on the 1st injection for only 48 hours and I actually do feel significantly better! Happier, full of energy, even after my lunchtime gym workout!." 
"I feel better overnight than I ever was on Androgel the entire year I was taking it." 
7) The Journal of Urology, May 1996, 155(5):1604-1608, "Improvement of Sexual Function in Testosterone Deficient Men Treated for 1 Year with a Permeation Enhanced Testosterone Transdermal System"; Journal of Andrology, 13(4):297-304, "A long-term, prospective study of the physiologic and behavioral effects of hormone replacement in untreated hypogonadal men"
10) The Journal of Clinical Endocrinology & Metabolism, Aug 1 2000, 85(8):2839-2853, "Transdermal Testosterone Gel Improves Sexual Function, Mood, Muscle Strength, and Body Composition Parameters in Hypogonadal Men"
13) http://www.cnbc.com/id/48149955, "Wall Street's Secret Weapon for Getting an Edge", Published: 11 Jul 2012, By: Cindy Perman
15) J Androl, 2003 Jul-Aug, 24(4):568-76, "Cognitive changes associated with supplementation of testosterone or dihydrotestosterone in mildly hypogonadal men: a preliminary report"
16) Neurology, Jul 10 2001, 57(1):80-88, "Testosterone supplementation improves spatial and verbal memory in healthy older men"