Prolactin is a hormone produced by the anterior pituitary gland. Its primary functions are to stimulate:
- Glandular breast tissue development
- Breast milk production
Control
Factors that stimulate prolactin production by the anterior pituitary gland include:
- Nipple stimulation (e.g., suckling by a baby)
- Thyrotropin-releasing hormone (TRH) from the hypothalamus
- Elevated oestrogen (e.g., during pregnancy)
- Stress
- Sleep
Progesterone inhibits the effects of prolactin on the breast tissue. This prevents breast milk production during pregnancy. After birth, progesterone levels fall rapidly, allowing prolactin to carry out its effects.
Dopamine (an important neurotransmitter) inhibits the secretion of prolactin.
CLINICAL RELEVANCE
Dopamine antagonists (e.g., antipsychotic medications) inhibit dopamine receptors, which can allow prolactin levels to rise, causing gynaecomastia (glandular breast tissue enlargement in males) and galactorrhea (breast milk production).
Function
Prolactin primarily affects the breast tissue, causing:
- Mammary alveoli growth (the small sacs within the lobules that produce and store breastmilk)
- Breastmilk synthesis by the epithelial cells of the alveoli
Prolactin inhibits the release of gonadotropin-releasing hormone (GnRH) by the hypothalamus. This inhibits the hypothalamic-pituitary-gonadal axis. Low GnRH means the anterior pituitary produces less LH and FSH, resulting in:
- Reduced stimulation of the testes in males, leading to low testosterone production
- Reduced stimulation of the ovaries in females, leading to absent ovulation and absent periods
CLINICAL RELEVANCE
Breastfeeding mothers have high levels of prolactin, causing anovulation (absence of ovulation) and amenorrhoea (absent periods). Exclusively breastfeeding is sometimes used as a relatively unreliable form of contraception.
Hyperprolactinaemia
Elevated prolactin levels can be caused by:
- Pregnancy and breastfeeding
- Prolactinomas (prolactin-secreting tumours of the pituitary gland)
- Hypothyroidism
- Medications (particularly dopamine antagonists)
In hypothyroidism, low thyroid hormones mean reduced negative feedback on the hypothalamus, resulting in elevated thyrotropin-releasing hormone (TRH). This elevated TRH stimulates excessive prolactin release by the pituitary.
Elevated prolactin causes symptoms of:
- Galactorrhea (breastmilk production not associated with pregnancy or breastfeeding)
- Menstrual irregularities, particularly amenorrhoea (absent periods)
- Infertility
- Reduced libido (low sex drive)
- Erectile dysfunction (in men)
- Gynaecomastia (in men)
Hypoprolactinaemia
Hypoprolactinaemia is a rare condition involving inadequate prolactin. It presents with failure to produce breastmilk in the postpartum period. Disorders affecting the pituitary can cause hypoprolactinaemia, including tumours, surgery, radiotherapy and Sheehan’s syndrome.
Sheehan’s syndrome is a rare complication of postpartum haemorrhage, where the drop in circulating blood volume leads to avascular necrosis of the pituitary gland. Low blood pressure and reduced perfusion of the pituitary gland lead to ischaemia in the pituitary cells and cell death. It only affects the anterior pituitary gland. Hormones produced by the posterior pituitary (oxytocin and antidiuretic hormone) are spared.
Medications
Dopamine agonists (e.g., cabergoline and bromocriptine) stimulate dopamine receptors and suppress the release of dopamine. They are used to treat hyperprolactinaemia.
Dopamine antagonists (e.g., antipsychotic medications) inhibit dopamine receptors, which allows for increased dopamine release and hyperprolactinaemia.
Last updated September 2024
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