The male hormone testosterone mediates the development of secondary male characteristics and contributes to hair growth, muscle mass, bone density, and erythropoiesis (red blood cell production.] Ninety-five percent of this androgen is produced by Leydig cells in the testes, in response to stimulation by luteinizing hormone (LH) from the anterior pituitary; the remaining 5% is produced in the adrenal glands0] Regulated by the gonadotropins (LH and follicle-stimulating hormone [FSH]) and modulated by the amount of sex hormone-binding globulin, testosterone levels can affect libido, erectile function, and spermatogenesis] Testosterone can act directly at the cellular level, or through its conversion to dihydrotestosterone by the enzyme 5a-reductase or to estradiol by the enzyme aromatase.

Plasma testosterone concentrations fluctuate due to its pulsatile secretion and to its diurnal rhythm, usually peaking at about 8 AM and falling to low levels at about 8 PM. Normally, free testosterone ranges between 9.0 and 30 ng/dL; total testosterone, between 300 and 1,200 ng/dL.

Whether the result of aging or of a disease process, the decline of testosterone (hypotestosteronemia) can lead to decreased libido, erectile dysfunction, decreased spermatogenesis, fatigue, depression, confusion, and hot flashes and night sweats.Persistently low testosterone levels can also be detrimental to organ systems other than the reproductive system (e.g., leading to osteoporosis and/or anemia).[13]
Symptomatic decline in male hormone levels are usually associated with hypogonadism. Classified as either primary or secondary, hypogonadism should be ruled out in any patient presenting with severe signs and symptoms of hypotestosteronemia or infertility.