The Endocrine System An Overview Susanne Hiller-Sturmhöfel, Ph.D., and Andrzej Bartke, Ph.D. A plethora of hormones regulate many of the body’s functions, including growth and development, metabolism, electrolyte balances, and reproduction. Numerous glands throughout the body produce hormones. The hypothalamus produces several releasing and inhibiting hormones that act on the pituitary gland, stimulating the release of pituitary hormones. Of the pituitary hormones, several act on other glands located in various regions of the body, whereas other pituitary hormones directly affect their target organs. Other hormone-producing glands throughout the body include the adrenal glands, which primarily produce cortisol; the gonads (i.e., ovaries and testes), which produce sex hormones; the thyroid, which produces thyroid hormone; the parathyroid, which produces parathyroid hormone; and the pancreas, which produces insulin and glucagon. Many of these hormones are part of regulatory hormonal cascades involving a hypothalamic hormone, one or more pituitary hormones, and one or more target gland hormones. KEY WORDS: endocrine function; hormones; hypothalamus; pituitary gland; gonad function; thyroid; parathyroid; pancreas; biochemical mechanism; biological feedback; biological regulation; hypothalamus-pituitary axis; pituitary-adrenal axis; pituitary-thyroid axis; literature review or the body to function properly, (i.e., within fractions of seconds) of composition of bodily fluids; and repro- its various parts and organs must information between different body duction. This article provides an Fcommunicate with each other to regions. Conversely, hormonal commu- overview of the hormone systems ensure that a constant internal environ- nication, which relies on the production involved in those regulatory processes. ment (i.e., homeostasis) is maintained. and release of hormones from various The article first summarizes some of the For example, neither the body temper- glands and on the transport of those basic characteristics of hormone- ature nor the levels of salts and minerals hormones via the bloodstream, is better mediated communication within the (i.e., electrolytes) in the blood must suited for situations that require more body, then reviews the various glands fluctuate beyond preset limits. Com- widespread and longer lasting regulatory involved in those processes and the munication among various regions of actions. Thus, the two communication the body also is essential for enabling systems complement each other. In SUSANNE HILLER-STURMHÖFEL, PH.D., the organism to respond appropriately addition, both systems interact: Stimuli is a science editor of Alcohol Health & to any changes in the internal and from the nervous system can influence Research World. external environments. Two systems the release of certain hormones and help ensure communication: the nervous vice versa. ANDRZEJ BARTKE, PH.D., is professor system and the hormonal (i.e., neuroen- Generally speaking, hormones con- and chairman of physiology at Southern docrine) system. The nervous system trol the growth, development, and Illinois University School of Medicine, generally allows rapid transmission metabolism of the body; the electrolyte Carbondale, Illinois. Vol. 22, No. 3, 1998 153 major hormones they produce. For molecular structures (e.g., size and or in the cell nucleus. The hormone- more in-depth information on those chemical properties). As a result of the receptor complexes then bind to certain hormones, the reader should consult structural differences, their mechanisms regions of the cell’s genetic material (i.e., endocrinology textbooks (e.g., Con- of action (e.g., whether they can enter the DNA), thereby regulating the activ- stanti et al. 1998; Wilson et al. 1998). their target cells and how they modulate ity of specific hormone-responsive genes. Finally, the article presents various the activity of those cells) also differ. Amino acid derivatives are modi- endocrine systems in which hormones Steroids, which are produced by the fied versions of some of the building produced in several organs cooperate gonads and part of the adrenal gland blocks of proteins. The thyroid gland to achieve the desired regulatory effects. (i.e., the adrenal cortex), have a molecular and another region of the adrenal The discussions focus primarily on structure similar to that of cholesterol. glands (i.e., the adrenal medulla) pro- the system responses in normal, healthy The molecules can enter their target duce this type of hormone (i.e., the people. For information regarding cells and interact with receptors in the amino acid derivatives). Like steroids, alcohol’s effects on some of the hormone fluid that fills the cell (i.e., the cytoplasm) amino acid derivatives can enter the systems, the reader is referred to sub- sequent articles in this issue of Alcohol Health & Research World. What Are Hormones? Hormones are molecules that are pro- Hypothalamus duced by endocrine glands, including Thyroid gland the hypothalamus, pituitary gland, Pituitary gland adrenal glands, gonads, (i.e., testes and ovaries), thyroid gland, parathyroid Parathyroid gland glands, and pancreas (see figure 1). The term “endocrine” implies that in response to specific stimuli, the products of those glands are released into the bloodstream.1 The hormones then are carried via the blood to their target cells. Some hormones have only a few specific target cells, whereas other hormones affect numerous cell types throughout the body. The target cells for each hormone are characterized by the presence of certain docking mole- cules (i.e., receptors) for the hormone Adrenal gland that are located either on the cell surface Pancreas or inside the cell. The interaction (Islets of between the hormone and its receptor Langerhans) triggers a cascade of biochemical reac- tions in the target cell that eventually modify the cell’s function or activity. Ovary (in female) Mechanisms of Action Testis Several classes of hormones exist, includ- (in male) ing steroids, amino acid derivatives, and polypeptides and proteins. Those hormone classes differ in their general 1Conversely, exocrine glands (e.g., sweat glands Figure 1 Schematic representation of the location of the major hormone-producing and salivary glands) release their secretions to the (i.e., endocrine) organs in the body. (For the purposes of illustration, both outside of the body (e.g., sweat) or into a hollow male and female endocrine organs are presented here.) space that is open to the outside (e.g., saliva released into the mouth). 154 Alcohol Health & Research World The Endocrine System cell, where they interact with receptor cell surface. The interaction initiates which are transported via the blood to proteins that are already associated biochemical changes in either the cell’s the pituitary gland. There, the releasing with specific DNA regions. The membrane or interior, eventually mod- hormones induce the production and interaction modifies the activity of ifying the cell’s activity or function. secretion of pituitary hormones, which the affected genes. in turn are transported by the blood to Polypeptide and protein hormones Regulation of Hormone Activity their target glands (e.g., the adrenal are chains of amino acids of various glands, gonads, or thyroid). In those lengths (from three to several hundred To maintain the body’s homeostasis glands, the interaction of the pituitary amino acids). These hormones are and respond appropriately to changes hormones with their respective target found primarily in the hypothalamus, in the environment, hormone produc- cells results in the release of the hormones pituitary gland, and pancreas. In some tion and secretion must be tightly that ultimately influence the organs instances, they are derived from inactive controlled. To achieve this control, targeted by the hormone cascade. precursors, or pro-hormones, which many bodily functions are regulated Constant feedback from the target can be cleaved into one or more active not by a single hormone but by several glands to the hypothalamus and pitu- hormones. Because of their chemical hormones that regulate each other itary gland ensures that the activity of structure, the polypeptide and protein (see figure 2). For example, for many the hormone system involved remains hormones cannot enter cells. Instead, hormone systems, the hypothalamus within appropriate boundaries. Thus, they interact with receptors on the secretes so-called releasing hormones, in most cases, negative feedback mech- anisms exist by which hormones released by the target glands affect the pituitary gland and/or hypo- thalamus (see figure 2). When certain predetermined blood levels of those Hypothalamus hormones are reached, the hypothala- mus and/or the pituitary ceases hormone release, thereby turning off the cascade. In some instances, a Releasing hormone so-called short-loop feedback occurs, Short-loop in which pituitary hormones directly feedback act back on the hypothalamus. The sensitivity with which these Anterior pituitary negative feedback systems operate (i.e., the target hormone levels that are required to turn off hypothalamic or pituitary hormone release) can Pituitary hormone change at different physiological states or stages of life. For example, the progressive reduction in sensitivity Target gland of the hypothalamus and pituitary to negative feedback by gonadal steroid hormones plays an important role in sexual maturation. Although negative feedback is more common, some hormone