40 Homeostasis, the tendency to maintain stability, is essential to survival. It is achieved by a system of control mechanisms. Endocrine control is achieved by biochemical regulators. Some of these are hormones, i.e. they are released from specialized glands into the blood to influence the activity of cells and tissues at distant sites. Others are paracrine factors, which are not released into the circulation, but which act on adjacent cells, e.g. in the regulation of the immune system. Finally, autocrine factors act on the very cells responsible for their synthesis. These different kinds of regulation are illustrated in Figure 40.1. Three broad classes are recognized: Many endocrine diseases arise from failure of control mechanisms (Table 40.1). Assessment of endocrine control presents particular difficulties. Table 40.1 Oversecretion Cushing’s disease where a pituitary adenoma secretes ACTH Undersecretion Primary hypothyroidism where the thyroid gland is unable to make sufficient thyroid hormone despite continued stimulation by TSH Failure of hormone responsiveness Pseudohypoparathyroidism where patients become hypocalcaemic despite elevated plasma PTH concentration because target organs lack a functioning receptor signalling mechanism
Endocrine control
Biochemical regulators
Hormone structure
Peptides or proteins. Most hormones fall into this class, although they vary enormously in size. For example, the hypothalamic factor thyrotrophin-releasing hormone has just three amino acids, whilst the pituitary gonadotrophins are large glycoproteins with subunits.
Amino acid derivatives. Examples include the thyroid hormones and adrenaline (epinephrine).
Assessment of endocrine control
Endocrine control
