An evaluation of the reproductive endocrine status is an essential component in the investigation of all male partners with either an abnormal physical examination suggestive of a disorder in testosterone production and action, an abnormal semen examination, or evidence of impaired sexual function. Initial laboratory assessment of the hypothalamic- pituitary-testicular axis includes the measurement of circulating levels of LH, FSH, and testosterone. The differential diagnosis and treatment of endocrine- dependent male-factor infertility is based on the history, physical examination, and reproductive hormone levels. Hypogonadotropic hypogonadism, also referred to as secondary hypogonadism, can occur as the congenital condition idiopathic hypogonadotropic hypogonadism (IHH). Androgen receptor abnormalities are rarely amenable to hormone therapy. Testosterone replacement therapy available in the United States includes oral, intramuscular, transdermal, and buccal preparations. A number of non-FDA-approved drugs and supplements are marketed as alternative therapies for declining androgens and decreasing libido and potency in older men.