Endocrine control
Biochemical regulators
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.
Hormone structure
Three broad classes are recognized:
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
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
