Endocrine System

The endocrine system primarily is associated with metabolic control of the body, including transport of substances through cell membranes and cellular metabolism for growth and secretion. By definition, hormones are chemical substances that are secreted by a cell (or cells) into bodily fluids which results in a physiological effect on other cells in the body. There are three basic types of hormones: steroids (e.g., estrogen, cortisol); derivatives of tyrosine, an amino acid (e.g., thyroxine); and proteins/peptides (e.g., oxytocin, insulin). The endocrine system changes over the course of the lifespan as a result of changes in amount of hormone secreted or sensitivity of target organs. This entry discusses hormones associated with the pituitary-thyroid axis, parathyroid, insulin, and adrenal cortical axis, and how they are altered in aging.