I have Monorchidism (One testicle in the scrotum). This condition can come in different forms and have different causes.
1) Undescended testicle
2) Vanished testicle (born with just one). This is my situation. I had exploratory surgery when I was a kid and they couldn't find one of them.
3-4% of live births of babies have one of the above forms of the problem (that seems un-believably high. I can't imagine a person working in the childbirth department of a hospital seeing 3 or 4 male babies born with Monorchidism, but on the other hand, it may not be very apparent at the time.
I noticed that I have been physically weak my whole life and that my testosterone levels has always been low. The expensive and painful androgen or testosterone shots are very helpful in that regard whenever I have gotten them (actually, I love it when the nurse gives me shot of the stuff in my hip).
Is there other men out there that have have a verified vanished testicle (unfindable through surgery) that have noticed a lack of energy? If so, I would like to know how you are doing.
Here's a Wikipedia link about the subject. There may be better sources than this, but this is all I have for now.
Note: I am still as kinky as all get out and fun in the sack. Yes, I need ED medicine, but even men with two testicles may need that when they are in their 50's.