Testosterone is the hormone required to make sperm and therefore is integral to sexual development, function, and interest. Testosterone also contributes to the growth of muscles and bones. It is produced by three glands: the hypothalamus, in the brain; the anterior pituitary gland, at the base of the skull; and the testicles.
As men age, testosterone levels tend to decline, which is normal. In fact, after age 30, most men begin to experience the symptoms associated with this gradual decrease. However, men of all ages may experience low testosterone due to other causes. The American Urological Association gauges low testosterone, or Low T, as less than 300 nanograms per deciliter.