Richard I. G. Holt - Essential Endocrinology and Diabetes

Figure 1.2 The sites of the principal endocrine glands. While the stomach, kidneys and duodenum are shown, a multitude of different hormones are secreted right the way along the gastrointestinal tract.

Traditionally, endocrinology has centred on specialized hormone‐secreting organs ( Figure 1.2), largely founded on the ‘endocrine postulates’ of Edward Doisy ( Box 1.3). While the focus of this textbook remains on these organs, virtually all tissues make hormones of some description or, equally relevant, modulate the action of hormones from other sites. All of these different aspects are important for a complete appreciation of endocrinology and its significance.

The gland must secrete a substance (an ‘internal secretion’)

Methods of detecting the secretion must be available

The purified substance (the hormone) must be recoverable from gland extracts

The hormone must be capable of isolation for its structure to be determined and for synthesis

To this could be added:

The hormone must act on specific target cells via a receptor such that excess or deficiency causes a specific phenotype

Endocrine (i.e. hormone‐secreting) cells may exist as distinct glands or be located as single cells within other organs, such as the gastrointestinal tract ( Table 1.2). The chapters in Part 2 are largely organized on this anatomical basis.

Hormones act by binding to specific receptors, either on the surface of or inside the target cell, to initiate a cascade of intracellular reactions, which frequently amplifies the original stimulus and generates a final response. These responses are altered in hormone deficiency or excess: for instance, GH deficiency leads to short stature in children, while excess causes over‐growth (either gigantism or acromegaly; Chapter 5).

Table 1.2 The endocrine organs and their hormones *

*The distinction between peptide and protein is somewhat arbitrary. Shorter than 50 amino acids is termed a peptide in this table.