Endogenous and exogenous

A substance is endogenous if it is produced naturally by the body. Contrast exogenous.

A substance can be endogenous or exogenous depending on the situation. For example, testosterone is naturally produced by testes and ovaries (endogenous), but it can also be injected into body as testosterone propionate (exogenous). The testosterone produced by the body is referred to as endogenous testosterone, whereas the testosterone metabolized from testosterone propionate is referred to as exogenous testosterone.