Cortisol: A major hormone produced by the adrenal cortex, which is the primary glucocorticoid secreted by the adrenal gland in response to ACTH stimulation or stress; cortisol has anti-inflammatory activity, and is involved in gluconeogenesis, glycogen storage in the liver, immune regulation, mediation of physiologic stress responses, Ca2+ absorption, secretion of gastric acid and pepsin, conversion of proteins to carbohydrates, and nutrient metabolism; it is secreted in a diurnal pattern–levels rise early morning, peak ± 8 am, and flatten in the evening, diurnal cycling is lost in Cushing syndrome; cortisol secretion is influenced by heat, cold, infection, trauma, excercise, obesity, intercurrent illness,