Introduction

Endocrinology is the study of hormones – chemical messengers secreted by endocrine glands and neurones. Hormones, usually travelling in the bloodstream, maintain homeostasis by acting on and coordinating activity within target organs or tissues.

Endocrinology is a rapidly growing field (Wilson 2005). Considerable progress is being made, with important implications for other areas of medicine, e.g. neuroendocrinology, which in turn has important applications within psychiatry. Advances are constantly being made and current research is adding to knowledge of genetic causes of many endocrine conditions. Genetic research is an area from which future treatments of endocrine disease will come.

Another area of considerable interest within endocrinology is the effects of cancer therapies on pituitary and thyroid function. One third of the UK population develops cancer, which is increasingly becoming a curable chronic disease; at the end of 2008 there were 2 million cancer survivors in the UK (3.3% of the total population and 10% of those over the age of 65). The overall survival following childhood cancer is 70–90% but 60–70% of these individuals have one or more ongoing medical problems and the most frequent conditions encountered are endocrine in nature.

Apart from diabetes mellitus (see pp. 149–175) and thyroid disorders, endocrine problems are not common. Many endocrine disorders are rarely seen and may be difficult to diagnose without complicated and exhaustive testing. As a result, patients may be referred by their general practitioner or local hospital to specialist centres for diagnosis. For the patient, this has the advantage of offering highly specialised care. It also means that relatively few nurses have the opportunity to care for people with these disorders. However, given the wide-ranging effects of endocrine dysfunction, it is important that all nurses have a general understanding of endocrinology. Endocrine diseases can affect every system of the body, sometimes causing disfigurement and a change in body image and quality of life (QoL) and even occasionally posing a threat to life. These disorders can seriously affect the patient’s psychological outlook, either as a direct result of the illness or by virtue of the individual’s reaction to it.

It is well documented that individuals react differently to being ill. Anger may be directed at family members or at nursing and medical staff. This situation may be exacerbated by an unstable mental state, mood swings, depression or frank psychosis, which may, in fact, be sequelae of the disorder, e.g. Cushing’s syndrome. The nurse must be aware of this possibility and react accordingly, offering support and understanding to the patient and their relatives. Identifying the patient’s worries and concerns, providing factual information and educating the patient about their condition and its management will all help to minimise psychological distress. Explanations that the underlying illness may be influencing the patient’s mood may help them and their relatives to cope.

As the tests and investigations required to diagnose some of the rarer endocrine conditions are often long and exhausting, clear explanations are essential so that the patient understands the need for them and the procedures that will be followed. This will help to reduce anxiety and increase the patient’s confidence in the health care team. The psychological support that the nurse can offer this group of patients is vitally important. Many benefit from referral to a support group (see Useful websites) and the contact this brings with other patients who have the same condition. Often these patients are young and probably otherwise in good health, and therefore the impact of endocrine disease on their body image and/or self-esteem should not be underestimated. For example, patients experiencing sexual dysfunction may feel embarrassed to talk to and seek support from family members and friends –in such cases, support from the health care professional is crucial.

Anatomy and physiology

The nature of the endocrine system, being one of control, means that it has far-reaching effects throughout the body on various target organs. This section provides a brief outline of hormonal action and outlines the anatomy and physiology of individual endocrine glands and structures (see Further reading, e.g. Montague et al 2005). The gonads, ovaries and testes are described in Chapters 7 and 8.

Hormones

The endocrine system is one of the two major control systems of the body, the other being the nervous system. The nervous system mediates endocrine activity. The endocrine glands/structures produce hormones, which are secreted into the bloodstream for transport to their respective target organs (Figure 5.1). Hormones are either lipid-based (e.g. glucocorticoids) or peptides (e.g. adrenaline [epinephrine], insulin).

Figure 5.1 The endocrine glands/structures and their location in the body.

Action

Hormones affect only those cells or organs upon which they have an excitatory or inhibitory action. This system allows individuals to respond to changes in their environment and is important in controlling growth and development, sexual maturation and homeostasis.

Many hormones are bound to proteins within the circulation. Only unbound or free hormones are biologically active, and that binding serves as a buffer against very rapid changes in plasma levels of a hormone. This principle is important in the interpretation of many tests of endocrine function.

Control

Most hormone systems are controlled by a negative feedback system which ensures that hormone levels, whilst fluctuating, remain within a preset range (see Appendix – normal values). Figure 5.2 illustrates how negative feedback operates in the hypothalamic–pituitary–thyroid axis.

Figure 5.2 Negative feedback regulation of the secretion of anterior pituitary hormones.

Pattern of secretion

Hormone secretion is either continuous or intermittent. An example of continuous secretion is thyroxine by the thyroid gland, in which hormone levels over a day, month or year show very little variation. Intermittent secretion is seen in three forms: circadian, menstrual and pulsatile.

Other factors which affect hormone secretion include stress, disease, trauma, surgery or emotional upset. As in any complex regulatory system, it is likely that disequilibrium in endocrine function will have important consequences. Disorders of the endocrine system may be categorised most simply as those involving overproduction (hypersecretion) and those involving underproduction (hyposecretion).

Pituitary

The pituitary gland lies immediately below the hypothalamus (part of the floor of the third ventricle of the brain) in the pituitary fossa. It is connected to the hypothalamus by the pituitary stalk and comprises two lobes which function independently of each other (Figure 5.3). During fetal life the glandular anterior lobe develops from the oropharynx to become the adenohypophysis. The neural posterior lobe is a down-growth from the forebrain, which becomes the neurohypophysis. The posterior pituitary receives nerve fibres from hypothalamic nuclei via the hypothalamohypophyseal tract in the pituitary stalk.

Figure 5.3 The pituitary gland and its relationship with the hypothalamus. ACTH, adrenocorticotrophic hormone; ADH, antidiuretic hormone; FSH, follicle-stimulating hormone; GH, growth hormone; LH, luteinising hormone; PRL, prolactin; TSH, thyroid-stimulating hormone.

(Reproduced from Waugh & Grant 2006 with permission.)

The hypothalamus contains specialised cells or nuclei (paraventricular and supraoptic) which synthesise the hormones antidiuretic hormone (ADH) (also known as arginine vasopressin [AVP] or vasopressin) and oxytocin. The hypothalamus also produces releasing/inhibiting hormones that regulate the secretion of anterior pituitary hormones (Figure 5.4). The hypothalamus also controls many centres for functions such as appetite, thirst, temperature regulation, sexual activity and sleep/wake cycles.

Figure 5.4 Schematic diagram showing some of the relationships between the hypothalamus, the anterior pituitary gland and their target organs. ACTH, adrenocorticotrophic hormone; CRH, corticotrophin-releasing hormone; GH, growth hormone; GHRH, growth hormone releasing hormone; GnRH, gonadotrophin-releasing hormone; LH (ICSH)/FSH, luteinising hormone/follicle-stimulating hormone; PRF, prolactin-releasing factor; PRL, prolactin; TRH, thyrotrophin-releasing hormone; TSH, thyroid-stimulating hormone.

The pituitary stalk carries blood to both lobes of the pituitary in a portal system by which the hypothalamic releasing/inhibitory hormones are carried to the anterior pituitary. The trophic hormones produced in the anterior pituitary stimulate the other endocrine glands. ADH and oxytocin are stored in the posterior pituitary until released. Table 5.1 outlines the hormones secreted by the pituitary gland.

Table 5.1 Hormones of the pituitary gland and their action

Hormone	Action
Anterior lobe
Growth hormone (GH) (syn. somatotrophin)	Pulsatile release. Does not have a single target gland but acts on a variety of tissues, e.g. bone, viscera and soft tissues. It is particularly important in children to stimulate growth. Action in adults recently thought to involve maintenance of cardiovascular activity, fat deposition and muscle development
Prolactin (PRL)	Main action is to stimulate lactation in females; also stimulates the corpus luteum of the ovary to secrete progesterone
Adrenocorticotrophic hormone (ACTH)	Secreted under circadian rhythm. Stimulates the adrenal cortex to secrete corticosteroids
Luteinising hormone (LH) known as interstitial cell-stimulating hormone (ICSH) in men (gonadotrophins)	In women LH promotes ovulation; stimulates formation of the corpus luteum. In men ICSH stimulates the production of the androgen hormone testosterone
Follicle-stimulating hormone (FSH) (a gonadotrophin)	FSH stimulates the production of sex hormones and contributes to regulation of the menstrual cycle in women. In women, stimulates the production of ovarian follicles and secretion of oestrogen; in men, stimulates spermatogenesis
Thyroid-stimulating hormone (TSH)	Stimulates the thyroid to release the hormone thyroxine

Posterior lobe
Antidiuretic hormone (ADH) (also known as arginine vasopressin [AVP] or vasopressin)	Controls water homeostasis in the body by regulating water reabsorption from the distal convoluted tubules of the kidney
Oxytocin	Induces uterine contraction during labour (unusually controlled by a positive feedback mechanism) and ejection of milk from breasts postpartum

The optic chiasm sits just above the pituitary fossa. Therefore, an expanding lesion from the pituitary or the hypothalamus may result in a defect in the visual fields because of pressure on the optic nerve or optic chiasm.

Thyroid

The thyroid is a gland in the neck sited anteriorly to the larynx and attached to the thyroid cartilages and the trachea. It comprises two lobes connected by an isthmus (Figure 5.5). Structures in close proximity include the oesophagus, the parathyroid glands, the recurrent laryngeal nerves and the carotid arteries.

Figure 5.5 The thyroid gland and associated structures.

The thyroid receives a rich blood supply from the superior thyroid arteries (branch of external carotid arteries) and from the inferior thyroid arteries (branch of subclavian arteries). Each lobe is filled with follicles, which produce and store two of the thyroid hormones. Between the follicles are parafollicular cells (C cells) which secrete the hormone calcitonin; this, together with parathyroid hormone (PTH) from the parathyroid glands, regulates calcium and phosphate homeostasis.

The hormones, thyroxine (T₄) and some circulating tri-iodothyronine (T₃), are synthesised and secreted by the thyroid gland. Thyroid hormone production is dependent on the availability of iodine in the diet (see Ch. 21). More T₄ than T₃ is produced, but T₄ is converted in some tissues to the more biologically active T₃. Over 99% of all thyroid hormone is bound to plasma thyroid-binding proteins. Only the free hormone is available for use by the tissues. If the levels of T₄ and T₃ in the blood fall, the hypothalamus releases thyrotrophin-releasing hormone (TRH). As the levels of TRH rise, the pituitary gland secretes thyroid-stimulating hormone (TSH) which stimulates the thyroid gland to produce more T₄ and T₃. The principal effect of thyroid hormones is to influence the metabolism of cells and, therefore, the metabolic rate. Low levels of hormone are associated with a low temperature, i.e. patients will complain of feeling cold, slow heart rate, weight gain, poor concentration and low mood whereas patients with high concentrations of thyroid hormones experience a rapid heart rate, heat intolerance, anxiety and weight loss despite increased appetite.

Adrenals

The adrenal glands are situated one on the upper part of each kidney (Figure 5.6A). They are highly vascular and derive their blood supply from the renal arteries, the inferior phrenic arteries and directly from the aorta; they are drained by the suprarenal veins.

Figure 5.6 A. The adrenal glands and associated structures. B. Layers through the adrenal gland.

The adrenal glands have a composite origin. The cortex, the outer part, has the same embryonic site of origin as the gonads. The medulla, the inner part, is derived from neural crest cells which have migrated from the developing neural tube and have become enclosed within the cortex. The latter can arguably be considered part of the nervous system. The secretions, and therefore the actions, of the two separate parts of the glands are quite different and will be discussed separately.

Adrenal cortex

The adrenal cortex has three layers (Figure 5.6B) that produce different hormones, collectively termed corticosteroids:

• mineralocorticoids (in the zona glomerulosa, the outer layer)

• glucocorticoids (in the zona fasciculata, the middle layer)

• adrenal sex hormones (androgens) and some glucocorticoids (in the zona reticularis, the inner layer).

Glucocorticoids

such as cortisol, the principal glucocorticoid, have varied and wide-ranging actions, many of which are not fully understood. They are essential to life and, in their absence, blood sugar, blood pressure and blood volume fall, sodium excretion increases and muscle contractility decreases. If present in excessive amounts, an opposite set of changes occurs: blood volume expands, blood pressure rises, potassium falls, glycogen storage is increased, blood sugar levels rise, connective tissue is reduced in quality and strength, and immunity is impaired. Wound healing and the inflammatory process are also inhibited.

Mineralocorticoids

The most important of the mineralocorticoids is aldosterone, which is the most potent regulator of sodium and potassium and is important in maintaining acid–base balance. Aldosterone acts on the distal convoluted tubules of the kidney and stimulates the cells to reabsorb and thus conserve sodium (see Chs 8, 20).

Adrenal androgens

The adrenal cortex, in addition to the gonads, produces male sex hormones. The effects are appropriate in the male, but in the female excessive production may have a virilising effect.

Adrenal medulla

The catecholamines adrenaline (epinephrine), noradrenaline (norepinephrine) and dopamine are secreted by the adrenal medulla. Eighty per cent of adrenomedullary secretion is adrenaline, most noradrenaline and dopamine being secreted by neurones and functioning as neurotransmitters.

In normal secretion, adrenaline probably has some effect on mean blood pressure; although it increases systolic pressure, it decreases diastolic pressure. When secreted under the control of the sympathetic nervous system (Ch. 9) it causes tachycardia, decreased gut motility and the closure of sphincters in the gut, pupil dilatation, bronchodilatation and piloerection.

Noradrenaline raises both systolic and diastolic blood pressure.

The difference between the effects of adrenaline and noradrenaline is partly explained by the presence of different receptors (α- and β-adrenoreceptors) on the surface of effector cells. Noradrenaline is most active at α-adrenoreceptors and adrenaline at β-adrenoreceptors.

The main effect of the catecholamines is on the cardiovascular system and the central nervous system, and on carbohydrate and lipid metabolism. Together, adrenaline and noradrenaline prepare the body for the ‘fight or flight’ response.

Disorders of the pituitary and hypothalamus

Disorders of the endocrine system result in either oversecretion or undersecretion of a hormone. Therefore, in principle, treatment should be straightforward, either replacing the missing hormone or removing, or suppressing the source of, the oversecretion, though in reality this is sadly not always the case. Advances have made the treatment of the wide range of endocrine disorders more successful, and the outlook for those affected is becoming ever more hopeful. However, patients with pituitary disease still have a greater mortality due to cardiovascular, cerebrovascular and respiratory causes than the general population and women have a worse outcome than men.

Hypersecretion of a hormone usually arises from a so-called ‘functioning’ pituitary adenoma. As this adenoma grows, it is likely to disrupt the normal function of other areas of the pituitary, and as a result progressive hypopituitarism occurs. This failure of the pituitary hormones occurs in a characteristic sequence (Boon et al 2006) – first GH is lost, followed by the gonadotrophins LH/ICSH and FSH, ACTH and then TSH. PRL deficiency is rare except as seen postpartum in Sheehan’s syndrome.

Hypersecretion of anterior pituitary hormones

Cushing’s disease caused by an excess of glucocorticoid hormones stimulated by an ACTH-producing adenoma of the pituitary is discussed in a later section (see p. 142).

Hypersecretion of growth hormone: acromegaly

Acromegaly is a rare condition caused by prolonged, excessive secretion of GH from a benign tumour of the pituitary gland occurring after fusion of the epiphyses of the long bones. It affects both sexes equally. The mean age at diagnosis is between 40 and 50 years, although excessive secretion of GH can occur at any age. In children where epiphyseal fusion has not yet occurred, it leads to gigantism. Acromegaly leads to a reduction in life expectancy with a two- to four-fold increase in mortality rate (Colao et al 2004).

Pathophysiology

Clinical features

The onset of acromegaly is insidious, leading on average to a delay in diagnosis of around 8 years from the onset of signs and symptoms, which include:

• excessive sweating, seen in >80% of patients

• headaches

• enlargement of the hands and feet

• tiredness and lethargy

• joint pain and osteoarthritis

• coarse facial features, frontal bossing, enlarged nose, protruding jaw and increased interdental separation

• oily skin

• deep voice

• tongue enlargement

• soft tissue swelling

• goitre and other organ enlargement

• features of hyperprolactinaemia (increased prolactin in the blood)

• visual field defects.

Complications include:

• hypertension

• insulin resistance, impaired glucose tolerance and diabetes mellitus

• obstructive sleep apnoea

• increased risk of colonic polyps and colorectal cancer

• ischaemic heart disease

• congestive cardiac failure

• cerebrovascular disease.

Common presenting signs/symptoms

The patient may notice an increase in shoe size or that they have had to have a ring enlarged. Along with their family and friends they may notice a change in their facial appearance. Women often feel very self-conscious about this, and also about their deepening voice, as they may be mistaken for men on the telephone. The major events that prompt a consultation with their GP are usually headache and sweating. Visual symptoms may be identified by their optometrist (ophthalmic optician), or their dentist may notice changes to dentition. Therefore patients may be referred to an endocrinologist by a number of routes.

Medical management

Investigations

Diagnosis can be confirmed following clinical examination and pituitary function testing. On clinical examination, some or all of the symptoms listed above may be present. Magnetic resonance imaging (MRI) usually shows the pituitary tumour and whether there is enlargement of the pituitary fossa. If acromegaly is suspected, a blood sample should be obtained and an insulin-like growth factor 1 (IGF-1) and random GH level measured. If the IGF-1 level is ‘normal’, when compared to the age-adjusted range, and GH <0.5 mU/L, the diagnosis of acromegaly can be excluded (Trainer 2002). Otherwise, the patient should proceed to have an oral glucose tolerance test (OGTT). The OGTT has been the gold standard for the diagnosis of acromegaly for many years. False positives may occur if the patient has diabetes mellitus, renal or liver disease, or anorexia nervosa. In acromegaly, measurement of GH levels during the OGTT are consistently elevated and fail to suppress to <2 mU/L in response to a 75 g oral glucose load. Normally, the level of GH would be undetectable. Acromegaly is therefore diagnosed on the basis of an elevated IGF-1 and failure to suppress GH during the OGTT. Further pituitary function tests may be performed to assess whether the remainder of the pituitary gland has been compromised. Prolactin is co-secreted in 30% of tumours.

Treatment

In untreated acromegaly, the mortality is nearly twice that of the normal population; early treatment is therefore essential. The aim is to relieve and, if possible, resolve symptoms associated with elevated GH and IGF-1 levels and the local effects caused by the tumour mass itself, and to prevent co-morbidity by lowering mean GH and IGF-1 levels to accepted ‘safe’ levels with minimal disruption to the patient’s lifestyle. Treatment may take the form of surgery, radiotherapy or medication. Until recently, surgery and radiotherapy were used in conjunction, with medical therapy then used to control residual excess GH secretion, as radiotherapy only lowers GH levels gradually. Today, medical therapy may also be used pre-operatively to control symptoms or to attempt to shrink a large tumour prior to surgery (Ben-Shlomo & Melmed, 2008). In patients unfit for surgery, medical therapy alone may be used.

Surgery

is currently regarded as the first-line therapy for acromegaly and, when performed by a pituitary surgeon, remains the treatment of choice. Hypophysectomy via the trans-sphenoidal route, which has a low morbidity and mortality, is the surgical procedure most commonly used. It results in a rapid fall in GH levels, often with no loss of any other pituitary hormones. Surgery is both rapid and inexpensive when compared to medical therapies. In patients with microadenomas <1 cm diameter it is curative in ~80% of cases. With macroadenomas >1 cm diameter it is nearer 40% (Kaltsas et al 2001).

Radiotherapy

Conventional external beam radiotherapy (25 fractions over 5 weeks delivered daily Monday–Friday) results in a delayed fall in GH and IGF-1 levels over the following decade which is paralleled by the development of hypopituitarism. After 5 years, 63% of patients have a normal IGF-1 (Jenkins et al 2006). Although used less often nowadays, it is safe, and radiation-induced visual damage is extremely rare. Patients may feel tired during the period of treatment and may experience some temporary hair loss at the site of entry of the beams. It is a useful treatment for patients in whom surgery is contraindicated or if GH/IGF-1 levels are not too high and/or there is a large tumour or residual tumour postoperatively, or where medical therapy fails to control GH/IGF-1 levels. Another form of radiotherapy is stereotactic radiotherapy, which may be single fraction radiosurgery or linear accelerator based stereotactic radiotherapy (multiple fraction). The expense is greater than conventional radiotherapy but is a ‘one-off’ cost. Long-term data are required, but early results suggest that GH and IGF-1 levels fall more rapidly after radiosurgery than with conventional radiotherapy and there is less damage to surrounding tissues, which may reduce the incidence of radiotherapy-induced hypopituitarism (Swords et al 2003). These two forms of radiotherapy can be given following conventional radiotherapy but cannot be used if the tumour is close to the optic chiasm.

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