DHT (Dihydrotestosterone) Levels
I cover the benefits of DHT (dihydrotestosterone), such as increased libido and orgasm frequency, in my page on
Testosterone and DHT.
Many men with a low sex drive are searching for a root cause and often they need to look no farther than their DHT levels. If your physician has never pulled your DHT levels, you can do it yourself here:
Inexpensive Testosterone Labs. (I have no affiliation with any of them.)
Once you have the number, then the question becomes "what do you do with it?" Let's look at how different experts choose target DHT levels. As you'll see, there is a considerable difference in opinion:
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What Is The Ideal DHT Level?
This is a tough question to answer, because it has been (as so many men's issues are) grossly understudied. A great deal of attention and
research has gone into testosterone levels, but the downstream hormones such as estradiol and DHT have definitely taken a back seat. This is
unfortunate, because, from what I have seen, you cannot ignore the effects of any hormone, especially one as powerful as DHT. Because of this,
there is a huge variability in philosophies and here are just a few examples:
1. Lab Ranges. Typical lab ranges for DHT are 16-79 or 30-85 ng/dl. Most physicians that pull your DHT will simply look look to see if you fall within this range to decide if your DHT levels are "okay" or not.
2. Life Extension Foundation. LEF is an organization that has tried to go a step beyond standard Western medicine and actually provide more natural longevity and anti-aging strategies. They are very research-based and their articles are generally excellent, although definitely pro-supplement in orientation. One nice service that they provide is a lot of lab testing and their recommended levels for DHT are between 30 and 50 ng/dl for adult males. (You should call them and confirm of course.)  Their reasoning for targeting the lower half of the physiological DHT range I am sure is avoidance of common DHT-related issues that some men could experience. DHT is the hormone most responsible for BPH (enlarged prostate) and male pattern baldness.
3. Anti-Aging Clinics. Some anti-aging clinics believe that all hormones should be restored to youthful levels. I call this a "replacement philosophy" and it a very popular theory out there that many men feel comfortable with. One argument for a replacement philosophy of DHT are a couple of studies that show that healthy men have no decrease in DHT through the normal aging process. For example, one study examined over 10,000 men and concluded that "the data show that after a peak of DHT:T in infancy and a subsequent decrease in puberty, the ratio of both androgens remains practically without change from approx. 20 years of age till old age."  Yet another study looked at 325 men over 40 that reported themselves in good or excellent health and found the following:
"Serum T, DHT and E(2) displayed no decrease associated with age among men over 40 years of age who self-report very good or excellent health although obesity and ex-smoking status were associated with decreased serum androgens (T and DHT) but not E(2). These findings support the interpretation that the age-related decline in blood T accompanying non-specific symptoms in older men may be due to accumulating age-related co-morbidities rather than a symptomatic androgen deficiency state." 
Assuming these studies are correct and the T/DHT ratio and the absolute level of DHT remain constant in healthy middle-aged and senior men, one appears to have Mother Nature on his side if he wants beefy DHT numbers. Yet another argument for this standpoint is the fact that so few men on HRT have issues with an enlarged prostate. When a man goes on testosterone therapy, his testosterone will ramp up (if the physician knew what he/she was doing) and DHT will go up correspondingly. This causes a small but measureable increase in prostate volume normally. I can say that, of all the men that have gone on HRT on
Peak Testosterone Forum, not one of them has complained of BPH that I remember. (It does happen from time to time though, so "eyes wide open.")
4. Supraphysiological. Interestingly enough, some of the topical (transdermal) testosterone gels can raise DHT significantly more than other forms of HRT (TRT). Many men on Androgel, for example, will end up with DHT well above the top of the range and quite a few even two times the of the lab range. The theory is that gels like these have a higher DHT conversion rate because of the extra alpha reductase in hair follicles. More concentrated products that are rubbed over a smaller surface area will not, according to the theory, have as high of a rate of conversion into DHT.
COMMENT: As you can see, there is tremendous variance in philosophy as far as treatment and safety when it comes to DHT. How can you decide, then, what ideal target DHT levels are?
Unfortunately, there are no DHT-mortality studies and very minimal research on DHT and cardiovascular / arterial health. Your crystal ball is probably just as polished as anyone else's at this point, and that means, at least at this time, you are going to have to do your own research and hopefully find a knowledgeable physician to provide their best guess.
Pulling DHT is not as common on the forum, because it is a fairly expensive test - closer to $100 than not. (In the U.S. you can now
pull total testosterone for less than $30 for example.) However, a few guys have pulled it and DHT is not a perfect predictor of libido and sex drive.
We had a man on HRT who have boosted his DHT above midrange and yet his libido really struggled.  He believed it was due to unmanaged estradiol and that is certainly a possibility. High prolactin can also lower libido, but that will generally drive down testosterone and, therefore, DHT simultaneously.
That said, DHT is one of the stronger predictors of libido, especially when it is low as this man described:
"The trick, of course, is to find a "normal" doc who'll prescribe anything unless you have extremely low T levels. My last few blood tests had me between 335-375 ng/dl, my libido was seriously diminished, and I was suffering noticeable mental fog and lack of concentration.." 
What was his DHT at the time? It was 27 ng/dl within a lab range of 16-79, i.e. near the bottom.
2) Neuro Endocrinol Lett, 2008 Apr, 29(2):201-4, "Dihydrotestosterone and testosterone throughout the life span of Czech men"
3) Clin Endocrinol (Oxf), 2012 Nov, 77(5):755-63, "Serum testosterone, dihydrotestosterone and estradiol concentrations in older men self-reporting very good health: the healthy man study"