The specialty of endocrinology deals with the endocrine organs. The endocrine system is composed of glandular organs that secrete their products directly into the blood. The products of the endocrine system are hormones, which are substances that have effects on various different target organs. Hormones allows the body to modify the function of multiple tissues at the same time through the adjustment of the circulating levels of just a few substances. In some respects, the endocrine system supplements the autonomic nervous system, or even acts as an effector component.

The body possesses several organs which either whole or in part have endocrine roles.

Other organs also possess some endocrine function but tend not to be regarded primarily as endocrine organs. For example, the kidney secretes renin (involved in the regulation of salt and water and blood pressure) and erythropoietin (stimulates the production of erythrocytes) and also activates vitamin D. Furthermore, the GI tract employs various hormones that help to co-ordinate the digestion of a meal.