A Brief History of Modern Endocrinology and Definitions of a True Hormone

Background and Objective: An overview of the history of endocrinology indicates that definitions of some initially developed concepts, including the term ‘hormone’ have been changed over time. This review provides a historical overview of current definitions of ‘hormone’ and the criteria of a true hormone. In addition, a brief history of hormone-related concepts and their transformation over time are discussed. Results: Classically, a hormone is a chemical substance secreted into the bloodstream and acts on distant tissues, usually in a regulatory fashion. Several newly discovered bioregulators and chemical signaling molecules are far from the classical definition of a true hormone and could not fulfill many relevant criteria. Major developments in the field of endocrinology accompanied by the complex terminology, currently used to describe hormonal actions of chemical messengers, underscore the need of the revision of such classical concepts. Conclusion: Complex terminology currently used to describe different hormonal actions of chemical messengers, suggests that it is time to conceptualize the term hormone and revise its classical definition.