Biology of Disease

tumors affecting other hormones is 15% GH, 10% ACTH, 4% FSH/LH with

less than 1% promoting TSH secretion. Large pituitary tumors may also exert

pressure on nerves, causing headaches and visual disturbances. Radiological

investigations such as X-rays, computer-aided tomography (CAT) scans and

magnetic resonance imaging (MRI), outlined in Chapter 18, are important in

locating tumors and estimating their sizes.

Prolactin secreting tumors, prolactinomas, cause hyperprolactinemia that, in

turn, can lead to infertility in both males and females. Nonprolactin secreting

tumors or pituitary stalk section by surgery, which block the dopaminergic

inhibition of prolactin secretion, also result in hyperprolactinemia.

Hyperprolactinemia abolishes menstruation (amenorrhea) and causes an

inappropriate release of breast milk (galactorrhea) and impotence and breast

development (gynecomastia) in males. Other causes of hyperprolactinemia

include drugs, for example phenothiazines, that block dopamine receptors, or

methyldopa that reduces the level of dopamine in the brain.

Investigating a patient with a possible prolactinoma includes assessing the

concentrations of prolactin in the plasma (Table 7.2) following stimulation with

TRH, although this test is not commonly used in most hospitals. In addition

to TRH, prolactin is also secreted in response to stress and estrogens. Patients

with a prolactinoma have plasma prolactin concentrations in excess of 2000

mU dm–3. These high values are generally not affected by TRH stimulation in

individuals with prolactinomas. The first line of treatment is with a dopamine

antagonist, such as bromocriptine, although surgical removal of the tumor

may be necessary in cases which do not respond to drug therapy.

Disorders of oxytocin are uncommon and have little clinical significance.

However, ADH release is essential for life and disorders of its release are

well recognized. The release of ADH is stimulated by increased osmolality

of the plasma and a decrease in blood volume detected by hypothalamic

osmoreceptors and cardiac baroreceptors respectively. The role of ADH in

fluid regulation is outlined in Chapter 8. A decreased output of ADH gives rise

to diabetes insipidus, characterized by excessive production of dilute urine

(polyuria). Patients are constantly thirsty (polydipsia), have hypernatremia

and a plasma osmolality in excess of 295 mmol kg–1.

X]VeiZg,/ DISORDERS OF THE ENDOCRINE SYSTEM

&+- W^dad\nd[Y^hZVhZ

[Prolactin] / mU dm–3 Interpretation

Males < 381 normal

Females < 629 normal

Males 381–1000 repeat test; does not usually indicate a serious problem

Females 629–1000 repeat test; does not usually indicate a serious problem

1000–2000 repeat test; the increased [prolactin] may be secondary to

stress, drug use, hypothalamic disorders, acromegaly, primary

hyperthyroidism or chronic renal failure

2000–4000 possible microprolactinoma or a hypothalamic disorder

4000–6000 prolactinoma probably present, although the possibilities of a

hypothalamic disorder or pregnancy should be investigated

> 6000 virtually always indicates the presence of a macroprolactinoma

Table 7.2Interpretation of serum prolactin concentrations in the diagnosis of hyperprolactinemia