It's often an upstream issue, and I'm not keen on people who suggest men in middle age (40s, 50s) should automatically jump to supplementing testosterone. I had low T for a while and it turned out it was due to a vitamin B deficiency (I'm a T2 Diabetic and my medication basically leeches vitamin B from my body, as well as blocking absorption). Started supplementing vitamin B and eating more foods with vitamin B and the next two tests over several months went from low, to the lower end of "normal" range, and seemed to be steadily increasing back to normal.