Testosterone is the major androgenic hormone. In males, 95 % of it is produced by interstitial (Leydig) cells and 5 % in the adrenal gland. In females, it is synthesized in the ovaries and the adrenal gland. Testosterone secretion is regulated by LH in males (negative feedback). It controls differentiation, the development of secondary male characteristics and, by acting together with FSH on Sertoli cells, spermatogenesis. Levels DECREASE after 70 years of age, and are also decreased if testicular Leydig cell function is impaired (associated with elevated levels of LH) and in gonadal insufficiency. Testosterone levels are HIGH in subjects taking testosterone and in hyperthyroidism. In females, this is an important assay in the investigation of hyperandrogenism (ovarian dystrophy, ovarian tumors or idiopathic hirsutism).In young boys, testosterone levels can be used to confirm early puberty. In adolescents, they can be used to establish the stage of puberty.