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Hyperprolactinaemia
Hyperprolactinaemia is the commonest disorder of the anterior pituitary. The upper limits of the normal range of the concetration of prolactin in the blood are 580 mIU/L in women and 450 mIU/L in men. At these limits there are both physiological and pathological causes of hyperprolactinaemia.
Physiological hyperprolactinaemia has the following causes.
- Pregnancy
- Breast feeding
- Nipple stimulation
- Post orgasm
- Sleep
- Stress
The pathological causes of hyperprolactinaemia include the following.
- Pituitary adenoma (prolactin secreting)
- Compression of the pituitary stalk
- Acromegaly
- Hypothyroidism
- Chronic renal failure
- Cirrhosis
- Post seizure
- Dopamine antagonists
Compression of the pituitary stalk can be caused by any pituitary adenoma, regardless of whether or not it also secretes prolactin. Hence, hyperprolactinaemia can be encountered in conditions such as acromegaly and Cushing's disease.
In hypothyroidism that is due to failure of the thyroid gland the hypothalamus will respond by increasing levels of thyrotropin releasing hormone. The TRH will stimulate the release of prolactin.
Prolactin
Prolactin is a single chain polypeptide hormone that contains 199 amino acids and has three disulphide bonds. It is made by the anterior pituitary.
Although the release of prolactin is stimulated by the secretion of thyroid releasing hormone by the hypothalamus, the dominant mechanism of control is inhibition by dopamine of the secretion of prolactin from the pituitary gland . As a consequence of this arrangement, any disease which disrupts the communication between the hypothalamus and pituitary gland via the pituitary stalk can cause hyperprolactinaemia.
As its name implies, prolactin is central to the process of making breast milk (lactogenesis). The process of lactation requires assistance from
oxytocin to allow the breast to release the milk which prolactin has induced it to synthesise.
The increase in prolactin that occurs during pregnancy promotes hyperplasia of the breasts.
A rise in prolactin after orgasm produces a sense of sexual gratification and opposes the sense of sexual arousal. Prolactin therefore contributes to the sexual refractory period. The high levels of prolactin which are found in early pregnancy and during lactation reduce the libido.
The levels of oestrogen in women and testosterone in men are reduced by prolactin. This accounts for the suspension of menstruation in women who are lactating and the contraceptive effect of breast feeding.
Clinical Features
Women who have hyperprolactinaemia may present with galactorrhoea, infertility, altered menstruation or amenorrhoea. The libido may be decreased and there can be vaginal dryness and dyspareunia.
Men can manifest infertility, decreased libido and impotence. Galactorrhoea affects up to one third. The antiandrogen actions of prolactin can reduce the quantity of facial hair and body hair.
In cases which are secondary to a piuitary tumour both men and women can develop features that are due to the
mass effect of the tumour.
Investigations
Hyperprolactinaemia can be confirmed by measuring the concentration of prolactin in the blood. Levels in excess of 3600mIU/L typically indicate a prolactin-secreting pituitary tumour or compression of the pituitary stalk.
The pituitary fossa should be imaged.
Other aspects of pituitary function should be evaluated.
Treatment
Bromocriptine is a dopamine agonist that can suppress prolactin secretion. It may also reduce the size of a prolactin-secreting pituitary adenoma.
Transphenoidal surgery may be employed to deal with a pituitary adenoma.