Endocrinology is the study of medicine that relates to the endocrine system, which is the system that controls hormones. An endocrinologist will deal with diseases that are caused by problems with hormones.
The endocrine system is a series of glands that produce and secrete hormones that the body uses for a wide range of functions. These control many different bodily functions, including: respiration, metabolism, reproduction, sensory perception, movement, sexual development and growth.
Hormones are produced by glands and sent into the bloodstream to the various tissues in the body. They send signals to those tissues to tell them what they are supposed to do. When the glands do not produce the right amount of hormones, diseases develop that can affect many aspects of life.