Most men know that DHT (dihydrotestosterone) can cause, or at least play a huge role, in two problems that men usually experience as they age: male pattern baldness and BPH (enlarged prostate). Neither one will kill you, but they sure can make me miserable. Men who produce low levels of DHT simply do not develop BPH. Furthermore, in puberty, it is well-accepted that DHT is the primary engine of prostate growth. So why not later in life as well? And why not when a man goes on HRT (TRT) due to the fact that some of his new testosterone will get converted into DHT.
One way to test out if DHT is really responsibel for the growth of the prostate later in life would be to simply give men DHT and see what happens. Or at least this would verify biological plauability. Surely, if DHT is the root cause of BPH, then some of the men would develop the condition, right? Researchers actually tested this situation and gave 70 mg of DHT per day to senior men and watched what happened. Interestingly, enough, their prostates did grow a little, but they could find no relationship with DHT:
"With time on study, there was an increase during 24 months in total prostate volume (29%; 95% confidence interval [CI], 23% - 34%), central prostate volume (75%; 95% CI, 64% - 86%; P < .01), and serum prostateï¿½specific antigen level (PSA; 15%; 95% CI, 6% - 24%). However, DHT had no effect on these changes (P > .2)." 
So, surprisingly, this study provided inconclusive results. Researchers, undoubtedly, had further doubts of the DHT-is-the-sole-cause of BPH by observing that only middle-age and senior men get BPH. Yet often they have lower testosterone and DHT levels. Furthermore, men on HRT (TRT) seem to seldom get BPH. It does happen, of course, but with much less frequency than what one might expect.
For example, we have had literally hundreds of men on HRT on the The Peak Testosterone Forum, and I do not remember one of them complaining of BPH. There probably has been someone, but my point is that it is quite rare. And this is really remarkable on our forum, because so many men are on testosterone cypionate in one form or the other and tend to have beefy testosterone levels. In fact, most of the guys on our forum using cypionate have doubled or tripled their testosterone levels from their hypogonadal state. So why aren't more of them miserable with an enlarged prostate?
Perhaps the answer is that cypionate does not raise DHT as much as the transdermal (topical) testosterones? This is generally true and something I discuss in my link on DHT Levels. Some of the transdermals can put a man over twice the top of the lab's physiological DHT range for example. So what happens when we look at the HRT (TRT) studies of transdermals on men? Do they show a higher-than-expected rate of BPH and prostate growth? Surprisingly, a number of studies actually show the opposite:
EXAMPLE 1: One stud of 1.0% testosterone gel produced very few case of BPH. This study looked at 56 (depressed) men and resulted in only one man with an enlarged prostate. 
EXAMPLE 2: Yet another study of 99 men and 106 men on 50 and 100 mg of Testim, respectively, yielded almost no cases of enlarged prostate. In fact, no men using 50 mg had BPH and only one using 100 mg. And, in the case of the latter, it was considered a mild case. 
Both of these two studies involve only 1 or 2% of the study population - that is not a very high incidence and matches what I have seen on the forum. These are simply not the kind of results one would expect if DHT was the sole factor in BPH.
CAUTION: Of course, talk to your doctor about his or her experience using HRT: the studies simply deal with averages. In addition, one study using stronger topicals did show a fairly significant rate of enlarged prostates. 
So how does one explain the fact that BPH is not more of an issue with men on HRT? One theory that has been thrown out there is that estradiol may play a major role in BPH. Basically, this theory is that in aging men estradiol has generally climbed while testosterone has fallen, leaving estrogen dominance in the prostate. And, according to some recent research, this extra estradiol can actually activate growth factors in the prostate, causing some men to struggle with an enlarged prostate.  More study work needs to follow though.
WHAT ABOUT DRUGS TO REDUCE DHT? Propecia (finasteride) has been shown "in clinical studies of men with BPH, 5-alpha reductase inhibitors reduce prostate volume by approximately 20% to 30%."  So one can rebut with the question, "If DHT has nothing to do with prostate volume, then why can you strink the prostate with DHT reducers. These drugs are risky in my opinion, because about 10% of men will have permanent side effects called "post finasteride syndrome." Essentially, a man develops low testosterone symptoms regardless of DHT or T levels - a mysterious situation indeed. But the incidence is high enough that many doctors have no doubt that it occurs and will not prescribe them to their patients.
NEWS FLASH: A recent study noted a link between male pattern baldness and prostate cancer.  Again, DHT has been implicated as playing a role in both, so this is not too surprising. See my page on Natural Prostate Cancer Prevention.
1) http://www.medscape.com/viewarticle/732585, "Dihydrotestosterone May Not Affect Prostate Growth But May Reduce BMD," Laurie Barclay, MD, Nov 16 2010
3) The Journal of Clinical Endocrinology & Metabolism, 85(8), Received: February 15 2000, Accepted: May 12 2000, "Transdermal Testosterone Gel Improves Sexual Function, Mood, Muscle Strength, and Body Composition Parameters in Hypogonadal Men"
4) Amer J of Psychiatry, Jan 2003, 160(1):105-111, "Testosterone Gel Supplementation for Men With Refractory Depression: A Randomized, Placebo-Controlled Trial"
5) The Journal of Clinical Endocrinology & Metabolism, 88(6), "AA2500 Testosterone Gel Normalizes Androgen Levels in Aging Males with Improvements in Body Composition and Sexual Function"