Ross & Wilson Anatomy and Physiology in Health and Illness (102 page)

A number of body tissues not normally described as endocrine glands secrete substances that act in tissues nearby (locally). Some of these are described below.

Histamine

This is synthesised and stored by mast cells in the tissues and basophils in blood. It is released as part of the inflammatory response, increasing capillary permeability and causing vasodilation. It also causes contraction of smooth muscle of the bronchi and alimentary tract and stimulates the secretion of gastric juice.

Serotonin (5-hydroxytryptamine, 5-HT)

This is present in platelets, in the brain and in the intestinal wall. It causes intestinal secretion and contraction of smooth muscle and its role in haemostasis (blood clotting) is outlined in
Chapter 4
.

Prostaglandins (PGs)

These are lipid substances found in most tissues. They act nearby and have potent and wide-ranging physiological effects in:

•
the inflammatory response

•
potentiating pain

•
fever

•
regulating blood pressure

•
blood clotting

•
uterine contractions during labour.

Other chemically similar compounds include
leukotrienes
, which are involved in inflammatory responses, and
thromboxanes
, e.g. thromboxane A
₂
, which is a potent aggregator of platelets. All of these active substances are found in only small amounts, as they are rapidly degraded.

Gastrointestinal hormones

Several local hormones, including gastrin, secretin and cholecystokinin (CCK), influence the secretion of digestive juices and their functions are explained in
Chapter 12
.

Endocrine disorders are commonly caused by tumours or autoimmune diseases and their effects are usually the result of:

•
hypersecretion (overproduction) of hormones

•
hyposecretion (underproduction) of hormones.

The effects of many of the conditions explained in this section can therefore be readily linked to the underlying abnormality.

Disorders of the pituitary gland

Learning outcomes

After studying this section you should be able to:

list the causes of diseases in this section

relate the features of conditions affecting the anterior pituitary to the actions of the hormones involved

relate the features of diabetes insipidus to abnormal secretion of antidiuretic hormone.

Hypersecretion of anterior pituitary hormones

Gigantism and acromegaly

The most common cause is prolonged hypersecretion of growth hormone (GH), usually by a hormone-secreting pituitary tumour. The conditions are only occasionally due to excess growth hormone releasing hormone (GHRH) secreted by the hypothalamus. As the tumour increases in size, compression of nearby structures may lead to:

•
hyposecretion of other pituitary hormones of both the anterior and posterior lobes

•
damage to the optic nerves, causing visual disturbances.

The effects of excess GH include:

•
excessive growth of bones

•
enlargement of internal organs

•
formation of excess connective tissue

•
enlargement of the heart and raised blood pressure

•
reduced glucose tolerance and a predisposition to diabetes mellitus.

Gigantism

This occurs in children when there is excess GH while epiphyseal cartilages of long bones are still growing, i.e. before ossification of bones is complete. It is evident mainly in the bones of the limbs, and affected individuals may grow to heights of 2.1 to 2.4 m, although body proportions remain normal (
Fig. 9.13
).

Figure 9.13 
Historical artwork showing effects of normal and abnormal growth hormone secretion.
From left to right: normal stature, gigantism (2.3 m tall) and dwarfism (0.9 m tall).

Acromegaly

This means ‘large extremities’ and occurs in adults when there is excess GH after ossification is complete. The bones become abnormally thick and there is thickening of the soft tissues. These changes are most noticeable as coarse facial features (especially excessive growth of the lower jaw), an enlarged tongue and excessively large hands and feet (
Fig. 9.14
).

Figure 9.14 
Facial features and large hands in acromegaly.

Hyperprolactinaemia

This is caused by a tumour that secretes large amounts of prolactin. It causes
galactorrhoea
(inappropriate milk secretion),
amenorrhoea
(cessation of menstruation) and sterility in women and impotence in men.

Hyposecretion of anterior pituitary hormones

The number of hormones involved and the extent of hyposecretion varies.
Panhypopituitarism
is absence of all hormones. Causes of hyposecretion include:

•
tumours of the hypothalamus or pituitary

•
trauma, usually caused by fractured base of skull, or surgery

•
pressure caused by a tumour adjacent to the pituitary gland, e.g. glioma, meningioma

•
infection, e.g. meningitis, encephalitis, syphilis

•
ischaemic necrosis

•
ionising radiation or cytotoxic drugs.

Ischaemic necrosis

Simmond’s disease
is hypofunction of the anterior pituitary gland, which only rarely affects the posterior lobe. The arrangement of the blood supply makes the gland unusually susceptible to a fall in systemic BP. Severe hypotensive shock may cause ischaemic necrosis of the gland. The effects include deficient stimulation of target glands and hypofunction of all or some of the thyroid, adrenal cortex and gonads. The outcome depends on the extent of pituitary necrosis and hormone deficiency. In severe cases, glucocorticoid deficiency may be life threatening or fatal. When this condition is associated with severe haemorrhage during or after childbirth it is known as
Sheehan’s syndrome
, and in this situation the other effects are preceded by failure of lactation.

Pituitary dwarfism (Lorain–Lévi syndrome)

This is caused by severe deficiency of GH, and possibly of other hormones, in childhood. The individual is of small stature but is well proportioned and mental development is not affected. Puberty is delayed and there may be episodes of hypoglycaemia. The condition may be due to genetic abnormality or a tumour.

Fröhlich’s syndrome

In this condition there is panhypopituitarism but the main features are associated with deficiency of GH, FSH and LH. In children the effects are diminished growth, lack of sexual development, obesity with female distribution of fat and learning disabilities. In a similar condition in adults, obesity and sterility are the main features. It may be the result of a tumour of the anterior pituitary and/or the hypothalamus but in most cases the cause is unknown.