Testosterone and DHT have metabolic and other effects, including Stimulating protein anabolism (increasing muscle mass and bone density) Stimulating renal erythropoietin production (increasing red blood cell mass) Stimulating bone marrow stem cells (modulating the immune system) Causing cutaneous effects (ie, sebum production, hair growth) Causing neural effects (ie, affecting cognition, increasing libido and possibly aggression) Testosterone also undergoes conversion to estradiol by the enzyme aromatase; estradiol mediates most of testosterone's action on organs such as bones and the brain.