Hormone structure, function and regulation

Before embarking on a discussion of the abnormalities affecting endocrine glands, a number of facets of the normal state will be highlighted.

Hormones are chemical messengers. They originate in one part of the body and act on other, often distant, parts. Hormones are selective in their action, influencing only those organs, tissues or cells that are receptive to them.

Although the variety and metabolic consequences of hormones are remarkable, they may be classified into three groups according to the tissues upon which they act. There are those that act directly on non-endocrine target tissues (effector hormones); those that control the synthesis and release of effector hormones (tropic hormones); and those that control the synthesis and release of tropic hormones (releasing hormones).

Hormones may also be classified by their molecular structure into three chemical groups; polypeptides, steroids and amino acid derivatives. This latter classification carries with it some of the fundamental concepts of biochemical mechanisms of hormone action.

Polypeptide hormones

Of the endocrine glands considered in this chapter, the hypothalamus, pituitary, endocrine pancreas and the parafollicular (C cells) of the thyroid produce polypeptide hormones. The hormones within this group vary tremendously in size, from simple molecules to complex polypeptides of up to 190 amino acid residues in length. However, the mechanism of action is basically the same, regardless of the size or amino acid composition of the hormone.

All act by binding to membrane receptors specific for the particular hormone.