DISORDERS OF THE ENDOCRINE SYSTEM
The endocrine system is one of two major control systems in the body, the other being the nervous system, that help control the activities of the body. It consists of a number of ductless glands (Figure 7.1) that produce hormones. Hormones are molecules that circulate in the blood and excite or inhibit the metabolic activity of target tissues or organs. These responses maintain and regulate body functions, such as growth and development, responses to stress and injury, reproduction, homeostasis and energy metabolism (Figure 7.2).
Hormones can be divided into three chemical groups: amines, peptides and proteins, and steroids (Table 7.1). Many amine hormones, such as adrenaline (epinephrine) and those produced by the thyroid gland, are derivatives of tyrosine. The majority of hormones are peptides and proteins, examples being insulin and growth hormone. A number of protein hormones, for example thyroid stimulating hormone, are glycoproteins in that they have carbohydrate groups covalently attached to them. All steroid hormones are derivatives of cholesterol and include cortisol and testosterone. Figure 7.3 shows examples of each type of hormones.