What causes hypogonadism?
Male hypogonadism is characterized by a deficiency of endogenous testosterone production resulting in abnormally low levels of circulating testosterone. Hypogonadism can be caused by a number of disorders, the most frequently observed being idiopathic hypogonadotrophic hypogonadism, hypopituitarism, Klinefelter’s syndrome, and late-onset hypogonadism. Most circulating testosterone (98%) is bound to transport proteins, with approximately 60% bound with high affinity to the sex hormone binding globulin (SHBG), and 38% weakly bound and transported by albumin.1 Only 2% of circulating testosterone is free and hence biologically active. Several lines of evidence suggest that not only free testosterone but also albumin bound testosterone is available to the target tissues, in case of an increased testosterone need. Therefore, the non-SHBG-bound testosterone is called "the bioavailable testosterone".