Overview of Endocrinology

How is it that humans and animals maintain quite constant blood concentrations of glucose throughout their lives despite wildly varying frequencies of meals? If your blood glucose concentration drops much below 1 mg per ml, your neurons will begin to misbehave, leading ultimately to coma and death. Yet skipping breakfast is rarely life-threatening.

The answer is that a battery of chemical messengers - hormones - are secreted into blood in response to rises and falls in blood glucose concentration and stimulate metabolic pathways that pull glucose concentrations back into the normal range.

Two systems control all physiologic processes:

• The nervous system exerts point-to-point control through nerves, similar to sending messages by conventional telephone. Nervous control is electrical in nature and fast.
• The endocrine system broadcasts its hormonal messages to essentially all cells by secretion into blood and extracellular fluid. Like a radio broadcast, it requires a receiver to get the message - in the case of endocrine messages, cells must bear a receptor for the hormone being broadcast in order to respond.

As will be repeatedly demonstrated, the nervous and endocrine systems often act together to regulate physiology. Indeed, some neurons function as endocrine cells.

Endocrinology is the study of hormones, their receptors and the intracellular signalling pathways they invoke. Distinct endocrine organs are scattered throughout the body. These are organs that are largely or at least famously devoted to secretion of hormones, and no introduction to endocrinology would be complete without some kind of endocrine organ "map" such as that below:

In addition to the classical endocrine organs, many other cells in the body secrete hormones. Myocytes in the atria of the heart and scattered epithelial cells in the stomach and small intestine are examples of what is sometimes called the "diffuse" endocrine system. If the term hormone is defined broadly to include all secreted chemical messengers, then virtually all cells can be considered part of the endocrine system.

A final introductory comment is warranted. Pursuit of an understanding of endocrinology is complicated by several of its principles:

• All pathophysiologic events are influenced by the endocrine milieu: There are no cell types, organs or processes that are not influenced - often profoundly - by hormone signaling.
• All "large" physiologic effects are mediated by multiple hormones acting in concert: Normal growth from birth to adulthood, for example, is surely dependent on growth hormone, but thyroid hormones, insulin-like growth factor-1, glucocorticoids and several other hormones are also critically involved in this process.
• There are many hormones known and little doubt that others remain to be discovered.

Consequently, endocrinology is presented here in two ways. First, the major endocrine organs and the hormones they produce are introduced, with delineation of major effects. Second, an integrated approach to understanding the multi-hormone control of several important phenomena is provided. Importantly, a considerable amount of endocrinology is incorporated into other sections. Gastrointestinal hormones, for example, are discussed throughout the section on pathophysiology of the digestive system.