Endocrine physiology is the study of hormones, the glands that produce them, and the effects that hormones have on their target organs. Endocrine function is necessary to maintain homeostasis, and is associated with the unconscious and subconscious functions of the body. It is closely linked with areas in the brain and nervous system that control homeostasis, especially the hypothalamus.

The main effects of hormones on the body are control of metabolism, nutrition and growth, sexual and reproductive development, and blood pressure and temperature control.

Traditionally, the gland releases the hormone into the bloodstream, the hormone travels over time to the target organ, and an effect is produced – a relatively slow process. It is now recognised that the physiology is much more complex. Hormones may be produced that act on neighbouring cells (e.g. histamine and prostaglandins), or they may even act on the secreting cell itself. These effects are much more rapid.

Classification of hormones

The more common hormones are listed in Figure EN1.

Polypeptides

Examples – vasopressin, oxytocin, prolactin, insulin, glucagon. These are usually produced as a prohormone which undergoes conversion to its active form. These hormones are stored in granules and secreted by exocytosis and thence into the bloodstream.

Glycoproteins

Examples – TSH, FSH, LH. These are polypeptide hormones linked to carbohydrate residues.

Steroids

Examples – corticosteroids, aldosterone, sex hormones. Steroids are synthesised in the cell mitochondria, from cholesterol.