Testosterone has a ubiquitous role in the male body and the importance of a decline in testosterone levels has wide ranging impact on: regulation of gonadal function, prostate development and growth, libido, cerebral function, behavior, mood, muscle mass, liver function, lipid regulation, bone formation, atherogenesis, erythropoiesis, hair growth and immune function.What the minimum required level of serum testosterone for the optimal health of each of these areas, nor whether each organ systems biological response to increasing or decreasing testosterone levels follows a dose-response, threshold or other behaviour is unclear.