Epi-testosterone, also known as epitestosterone, is a natural steroid hormone found in the human body. It is structurally similar to testosterone, one of the primary male sex hormones. Epi-testosterone is produced in small quantities and is typically considered inactive in terms of its androgenic effects, meaning it does not have the same masculinizing or anabolic properties as testosterone.

One of the notable aspects of epi-testosterone is its role in anti-doping testing in sports. In many cases, the ratio of testosterone to epi-testosterone in a person's urine is measured as part of anti-doping protocols. This is because some athletes have artificially elevated testosterone levels due to doping, which can disrupt the natural balance between testosterone and epi-testosterone. An elevated testosterone-to-epi-testosterone ratio can be an indicator of doping or the use of performance-enhancing substances.

In essence, epi-testosterone is a naturally occurring hormone that serves as a reference point for evaluating the integrity of the endocrine system and detecting potential doping in sports.