Ross & Wilson Anatomy and Physiology in Health and Illness (103 page)

Disorders of the posterior pituitary

Diabetes insipidus

This is a relatively rare condition usually caused by hyposecretion of ADH due to damage to the hypothalamus by, for example, trauma, tumour or encephalitis. Occasionally it occurs when the renal tubules do not respond to ADH. Water reabsorption by the renal tubules is impaired, leading to excretion of excessive amounts of dilute urine, often more than 10 litres daily, causing dehydration, extreme thirst (polydipsia). Water balance is disturbed unless fluid intake is greatly increased to compensate for excess losses.

Disorders of the thyroid gland

Learning outcome

After studying this section you should be able to:

compare and contrast the effects of hyperthyroidism and hypothyroidism, relating them to the actions of T
₃
and T
₄
.

These fall into three main categories:

•
abnormal secretion of thyroid hormones (T
₃
and T
₄
)

    
–
hyperthyroidism

    
–
hypothyroidism

•
goitre – enlargement of the thyroid gland

•
tumours.

Abnormal thyroid function may arise not only from thyroid disease but also from disorders of the pituitary or hypothalamus; in addition, insufficient dietary iodine causes deficiency in thyroid hormone production. The main effects are caused by an abnormally high or low basal metabolic rate.

Abnormal secretion of thyroid hormones

Hyperthyroidism

This syndrome, also known as
thyrotoxicosis
, arises as the body tissues are exposed to excessive levels of T
₃
and T
₄
. The main effects are due to increased basal metabolic rate (see
Table 9.3
).

In older adults, cardiac failure is another common consequence as the ageing heart works harder to deliver more blood and nutrients to the hyperactive body cells. The main causes are:

•
Graves’ disease

•
toxic nodular goitre

•
adenoma (a benign tumour,
p. 224
).

Graves’ disease

Sometimes called
Graves’ thyroiditis
, this condition accounts for 75% of cases of hyperthyroidism. It affects more women than men and may occur at any age, being most common between the ages of 30 and 50 years. It is an autoimmune disorder in which an antibody that mimics the effects of TSH is produced, causing:

•
increased release of T
₃
and T
₄
and signs of hyperthyroidism (see
Table 9.3
)

•
goitre (visible enlargement of the gland,
Fig. 9.16
) as the antibody stimulates thyroid growth

•
exophthalmos in many cases.

Exophthalmos

This is protrusion of the eyeballs that gives the appearance of staring, which is due to the deposition of excess fat and fibrous tissue behind the eyes (
Fig. 9.15
); it is often present in Graves’ disease. Effective treatment of hyperthyroidism does not completely reverse exophthalmos, although it may lessen after 2 to 3 years. In severe cases the eyelids become retracted and may not completely cover the eyes during blinking and sleep, leading to drying of the conjunctiva and predisposing to infection. It does not occur in other forms of hyperthyroidism.

Figure 9.15 
Abnormally bulging eyes in exophthalmos.

Toxic nodular goitre

In this condition one or two nodules of a gland that is already affected by goitre (see
Fig. 9.16
) become active and secrete excess T
₃
and T
₄
causing the effects of hyperthyroidism (
Table 9.3
). It is more common in women than men and after middle age. As this condition affects an older age group than Graves’ disease, arrhythmias and cardiac failure are more common. Exophthalmos does not occur in this condition.

Hypothyroidism

This occurs when there is insufficient T
₃
and T
₄
secretion causing:

•
congenital hypothyroidism in children

•
myxoedema in adults.

Congenital hypothyroidism

Previously called cretinism, this is a profound deficiency or absence of thyroid hormones that becomes evident a few weeks or months after birth. Hypothyroidism is endemic in parts of the world where the diet is severely deficient in iodine and contains insufficient for synthesis of T
₃
and T
₄
. Absence of thyroid hormone results in profound impairment of growth and mental development. Unless treatment begins early in life, mental impairment is permanent and the individual typically has disproportionately short limbs, a large protruding tongue, coarse dry skin, poor abdominal muscle tone and, often, an umbilical hernia.

Myxoedema

This condition is prevalent in the elderly and is five times more common in females than males. Deficiency of T
₃
and T
₄
in adults results in an abnormally low metabolic rate and other effects shown in
Table 9.3
. There may be accumulation of polysaccharide substances in the subcutaneous tissues, especially of the face. The commonest causes are: autoimmune thyroiditis, severe iodine deficiency (see goitre) and healthcare interventions, e.g. antithyroid drugs, surgical removal of thyroid tissue or ionising radiation.

Autoimmune thyroiditis

The most common cause of acquired hypothyroidism is
Hashimoto’s disease
. It is more common in women than men and, like Graves’ disease, an organ-specific autoimmune condition. Autoantibodies that react with thyroglobulin and thyroid gland cells develop and prevent synthesis and release of thyroid hormones causing hypothyroidism. Goitre is sometimes present.

Simple goitre

This is enlargement of the thyroid gland without signs of hyperthyroidism. It is caused by a relative lack of T
₃
and T
₄
and the low levels stimulate secretion of TSH resulting in hyperplasia of the thyroid gland (
Fig. 9.16
). Sometimes the extra thyroid tissue is able to maintain normal hormone levels but if not, hypothyroidism develops. Causes are:

•
persistent iodine deficiency. In some parts of the world where there is dietary iodine deficiency, this is a common condition known as
endemic goitre

•
genetic abnormality affecting synthesis of T
₃
and T
₄

•
iatrogenic, e.g. antithyroid drugs, surgical removal of excess thyroid tissue.

The enlarged gland may cause pressure damage to adjacent tissues, especially if it lies in an abnormally low position, i.e. behind the sternum. The structures most commonly affected are the oesophagus, causing dysphagia; the trachea, causing dyspnoea; and the recurrent laryngeal nerve, causing hoarseness of voice.

Tumours of the thyroid gland

Malignant tumours are rare.

Benign tumours

Single adenomas are fairly common and may become cystic. Sometimes the adenoma secretes hormones and hyperthyroidism may develop. The tumours may become malignant, especially in the elderly.

Disorders of the parathyroid glands