While a small percentage of testosterone is made in the adrenal glands, about 95 percent of testosterone is produced in male testicles. The hypothalamus detects a testosterone deficiency and sends a gonadotropin-releasing hormone to the pituitary gland, which then sends two hormones to the testicles that stimulate testosterone production.
Testosterone is a 19-carbon steroid hormone made from cholesterol. Pure or free testosterone, unbound to proteins, makes up about 2 to 3 percent of total testosterone levels. In this form, it is able to enter cells and activate receptors.
Testosterone bound to sex-hormone-binding-globulin, a protein produced in the liver, is biologically inactive. It does not provide any of the benefits of pure testosterone, but makes up about 40 to 50 percent of the total testosterone levels. The remainder of testosterone is bound to albumin, another protein produced in the liver. Although albumin-bound testosterone is also inactive, the bond between protein and testosterone is weak, and can be broken to create free testosterone.
A male begins producing testosterone about seven weeks after conception. It aids in forming male genitals in the womb, and surges in the body during puberty. Testosterone drives the growth of body hair, plays a role in behaviors such as aggression or dominance, and can spark competitiveness. Low testosterone can result in a loss of confidence or lack of motivation.