Testosterone is integral to normal sexual development and function for everyone, though it is most prominent in males. Developed primarily in the testes in men—the adrenal glands produce a small amount, too—testosterone can cause problems if it is deficient. Around the age of 30, men's testosterone levels slowly start to drop. By the time they reach their 60s, 35 percent of men have lower testosterone levels than younger men.

People with low testosterone levels can experience symptoms such as fatigue, reduced lean muscle, irritability, depression, obesity, loss of body hair, low sex drive and erectile dysfunction (ED).

But what are the causes of low testosterone?

How does testosterone production work?

A hormonal axis centered around the pituitary gland is responsible for testosterone production and regulation.

"The pituitary is a small, roughly pea-sized gland in the brain which secretes stimulatory hormones called gonadotropins.