I'm going to disagree wit Dr. Rynne's comment. If it is causing you pain, get it taken care of by a urologist before it becomes unmanageable. I speak from personal experience. Doctors told me it was nothing to worry about. When the pain became so great I could no longer work surgery was tried. No luck. Bottom line was more surgery and eventual loss of the testicle.
I never had my testosterone level checked during that time but I felt fine and had a good sex life. Testosterone levels vary from man to man. What may be considered low by your doctor may be normal for you. Before going on testosterone supplements consult an endocrine specialist.
Varicocele and what it does to men remains a controversial subject. There are those who say that varicocele are harmless and I would agree with this. I have seen hundreds of varicoceles as a vasectomist and they never seem to cause any problems. The best way to know if you have sufficient testosterone is not by blood test but by how you feel.
If your testosterone levels are low you will have low libido, low energy and drive, low mood.