Hyperprolactinaemia is the most common pituitary cause of amenorrhoea. There are many causes of a mildly elevated serum prolactin concentration, including stress and a recent physical or breast examination. If the prolactin concentration is greater than 1000 milliunits per litre (mu/l), then the test should be repeated and, if still elevated, it is necessary to image the pituitary fossa with a CT or MRI scan (Figure 12.1). Hyperprolactinaemia may result from a prolactin-secreting pituitary adenoma or from a nonfunctioning ‘disconnection’ tumour in the region of the hypothalamus or pituitary, which disrupts the inhibitory influence of dopamine on prolactin secretion. Large nonfunctioning tumours are usually associated with serum prolactin concentrations of less than 3000 mu/l, while prolactin-secreting macroadenomas usually result in concentrations of 8000 mu/l or more. Other causes include hypothyroidism, PCOS (up to 2500 mu/l) and several drugs (e.g. the dopaminergic antagonist phenothiazines, domperidone and metoclopramide).

In women with amenorrhoea associated with hyperprolactinaemia, the main symptoms are usually those of estrogen deficiency. In contrast, when hyperprolactinaemia is associated with PCOS, the syndrome is characterised by adequate estrogenisation, polycystic ovaries on ultrasound scan and a withdrawal bleed in response to a progestogen challenge test. Galactorrhoea may be found in up to one-third of women with hyperprolactinaemia, although its appearance is correlated neither with prolactin levels nor with the presence of a tumour. Approximately 5% of patients present with visual field defects.