what is an endogenous substance

“Endogenous” means “growing or originating within an organism.” An endogenous substance, therefore, is a substance that originates within the body of a living organism.

Lots of people use endogenous to mean “inside the body,” or “caused by factors within the body.” Endogenous and endogenous substance are words that are used relatively frequently in medical parlance by clinicians, but aren’t commonly used by people outside the medical field.

Endogenous is the opposite of exogenous, which means originating outside a living organism.

Here are several examples of endogenous substances (all of which, as you can tell by their names, originate within the body):

Endogenous cholesterol. If you’ve paid any attention to cholesterol drug commercials over the years, you might remember one commercial that talked about the two sources of cholesterol: dietary cholesterol, and cholesterol originating in your own body. That second source of cholesterol — produced by your own liver inside your body — makes that cholesterol an endogenous substance.
Endogenous opioids. In some cases, your body manufactures its own pain relief in the form of endogenous opioid compounds. These compounds work just like opioid medications you take, blocking pain you otherwise would feel. Medical experiments have shown that your brain actually activates these self-manufactured drugs to prevent pain in certain cases — in fact, this is the physical mechanism behind the “runner’s high.”
Endogenous autoantibodies. Your immune system is designed to fight against foreign invaders like viruses and bacteria. Sometimes, though, it makes antibodies that mistakenly attack your own organs and other tissues. These antibodies are endogenous autoantibodies — endogenous because they originate within your body, and “auto” antibodies because they’re attacking their own organism. These endogenous substances lead to autoimmune diseases such as celiac disease (where the endogenous autoantibodies attack the small intestine) and type 1 diabetes (where they attack the pancreas).
Endogenous hydrogen sulfide. Chronic obstructive pulmonary disease (COPD) is characterized by progressive restriction of your airways. Put simply, the disease makes it harder and harder to breathe. Medical researchers have investigated what triggers this problem, and have pinpointed several potential culprits. One of these is endogenous hydrogen sulfide, or hydrogen sulfide originating in your own body. One study measured the levels of hydrogen sulfide in the blood of people with COPD, and found that higher levels correlated with worse cases of COPD. The researchers concluded that endogenous hydrogen sulfide is involved in COPD.