The endocrine system consists of cells that produce hormones. Hormones are chemical substances that are formed in the body and carried in the bloodstream to have a specific regulatory effect on the activity of other organs or cells in the body. For example, a component of the pancreas is made up of specialized cells clustered together in islands within the organ, called islets of Langerhans. These cells produce multiple hormones, the most critical one being insulin, which is a substance that helps control the amount of sugar in the blood.

An endocrine tumor is a growth that affects the parts of the body that secrete hormones. Because an endocrine tumor arises from cells that produce hormones, the tumor itself can produce hormones and cause serious illness.

In addition, a neuroendocrine tumor originates from the hormone-producing cells, specifically those cells of the neuroendocrine system. The neuroendocrine system consists of cells that are a cross between traditional endocrine cells (or hormone-producing cells) and nerve cells. Neuroendocrine cells are found throughout the body in organs, such as the lungs and gastrointestinal tract, and perform specific functions, such as regulating the air and blood flow through the lungs and controlling the speed at which food is moved through the gastrointestinal tract.