Testosterone is an important male hormone. In men, it’s mostly produced in the testicles.

Testosterone is responsible for building muscles and bone mass, and for stimulating sperm production. Your testosterone levels also factor into your sex drive.

Normal testosterone levels will vary. However, adult men are considered to have low testosterone, or low T, when their levels fall below 300 nanograms per deciliter (ng/dL), according to guidelines from the American Urological Association (AUA).

When your testosterone levels decrease, your desire for sex also decreases.

Decreasing testosterone is a normal part of aging. However, a drastic drop in testosterone can lead to decreased libido.

Talk to your doctor if you think this might be an issue for you. You may be able to take supplements or gels to increase your testosterone levels.