Although each ductless gland (Fig. 16.1) produces a hormone with specific functions, there is an integrated relationship between the activities of the glands. If these relationships are disturbed by a disease process, the consequences can be far reaching.

Fig. 16.1 The endocrine glands.

(After Waugh A, Grant A 2001 Ross and Wilson anatomy and physiology in health and illness, 9th edn. Churchill Livingstone, Edinburgh. With permission of Elsevier.)

The hypothalamus and the pituitary gland together form the central control unit for the production and secretion of many hormones. The hypothalamus produces ‘releasing’ and ‘inhibiting’ hormones that influence the anterior lobe of the pituitary gland to release corresponding hormones, the trophic hormones, some of which affect target cells in the body directly while others do so through the intermediary of a second endocrine organ such as the thyroid, the adrenal cortex and the gonads. The second endocrine organ, in turn, produces a third specific hormone such as levothyroxine sodium (thyroxine sodium), corticosteroids or sex hormones, which influences various other body functions.

Chemoreceptors in the hypothalamus register the blood hormone level and react accordingly by either increasing or decreasing hormone production. If the blood hormone level is low, more hormone is secreted; if the blood hormone level is high, hormone production is reduced (negative feedback; see Fig. 16.2). Not all hormonal activities, however, follow this mechanism.

Fig. 16.2 Negative feedback mechanism.

HYPOTHALAMUS AND PITUITARY GLAND

The hypothalamus forms the base of the brain (diencephalon). It is responsible for the coordination of nervous and endocrine systems and therefore many basic life functions such as cardiovascular, respiratory and alimentary functions, sexual behaviour and reproduction. It is connected to the pituitary gland, a small endocrine gland of great importance, by the hypophyseal stalk or infundibulum. The pituitary gland lies almost completely surrounded by bone in the base of the skull. It consists of three parts: the anterior lobe (adenohypophysis), the median eminence and the posterior lobe (neurohypophysis).

The anterior pituitary (see Fig. 16.3) synthesises and releases seven known trophic hormones. These are somatotrophin, which is involved in prepubertal body growth; prolactin, which stimulates the growth and secretory activity of the female breasts during pregnancy; melanocyte-stimulating hormone, which causes an increase in cutaneous pigmentation; adrenocorticotrophic hormone (ACTH), which governs the secretions of some of the hormones by the adrenal cortex; thyrotrophin (thyroid-stimulating hormone, TSH), which stimulates thyroid activity; follicle-stimulating hormone or human menopausal gonadotrophin, which stimulates growth of ovarian follicles and secretion of oestrogen in the female and spermatogenesis in the male; and luteinising hormone, which stimulates the production of progesterone in the corpus luteum of the follicle (female) and activates androgen secretion by the Leydig cells of the testis (male).

Fig. 16.3 Hormones released by anterior pituitary.

The posterior pituitary (see Fig. 16.4) hormones vasopressin (antidiuretic hormone, ADH) and oxytocin are produced in the hypothalamus and secreted (neurosecretion) directly into the bloodstream of the infundibulum and posterior lobe of the pituitary gland, from where they can be released into the body.