What is being tested?
Prolactin is a 23kD sized hormone produced by the lactotroph cells of the pituitary gland, a grape-sized organ found at the base of the brain. Normally present in low amounts in men and non-pregnant women, prolactin's main role is to promote lactation (breast milk production).
Prolactin levels increase throughout pregnancy with peak levels being found at delivery and just after childbirth. During pregnancy prolactin, oestrogen and progesterone stimulate breast milk development. Following childbirth, prolactin helps initiate and maintain the breast milk supply. If a woman does not breastfeed, her prolactin soon drops back to low pre-pregnancy levels. If she does breastfeed, suckling by the infant plays an important role in the production of prolactin. When the baby feeds, this has an effect on the amount of prolactin secreted by the pituitary, and this is turn controls the amount of milk produced.
By around six weeks after delivery, prolactin may return to pre-pregnancy levels even in mothers still feeding infants. This mirrors oestradiol production which falls at the same time. Prolactin response to breast feeding decreases over time, when after months of feeding, prolactin elevation is minimally increased above pre-pregnancy levels.
Besides pregnancy, the most common cause of raised prolactin levels are certain prescribed medications and prolactinomas; prolactin-producing tumours of the pituitary gland. Prolactinomas (lactotroph adenomas) are the most common type of pituitary tumour and are usually benign. They develop more frequently in women but are also found in men.
Elevations of prolactin may be minimal or extreme in prolactinomas, being up to many thousands of times in relation to the reference range. Other causes of prolactin elevation are usually not extreme, such as with stress, breast feeding, pregnancy and certain drug administration. Occasionally, pituitary adenomas are comprised of two types of cells, lactotroph and somatotroph and can produce growth hormone in conjunction with prolactin. Some adenomas can also occur in a MEN Type 1 (Multiple Endocrine Neoplasia) syndrome.
Problems can arise both from the unintended effects of excess prolactin, such as milk production in the non-pregnant woman (and rarely, man) and from the size and location of the tumour. If the pituitary gland and/or the tumour enlarge significantly it can put pressure on the optic nerve, causing headaches and visual disturbances; and it can interfere with the other hormones that the pituitary gland produces.
In women, prolactinomas can cause infertility and irregular periods; in men, these tumours can cause a gradual loss in sexual function and desire. If left untreated, prolactinomas may eventually damage tissues surrounding them.
How is it used?
Prolactin levels are used with other tests, to help:
When is it requested?
Prolactin may be requested when a patient has symptoms of a prolactinoma such as unexplained headaches, visual impairment and/or galactorrhoea. They may also be used, along with other tests, when a woman is experiencing infertility or irregular periods; or when a man has symptoms such as: a decreased sex drive, galactorrhoea or infertility. Prolactin levels are also often ordered in men as a follow-up to a low testosterone level.
When a patient has a prolactinoma, prolactin levels may be used to monitor the growth of the tumour and its response to treatment. They may also be used at regular intervals to monitor for prolactinoma recurrence.
Prolactin levels may be used with other hormone levels such as growth hormone, when your doctor suspects that you have more general hypopituitarism (low levels of pituitary function which result in lowered levels of hormones being produced). Prolactin levels may also be monitored when you have a condition or are taking medications that affect dopamine (a brain chemical that controls the production of prolactin).
What does the result mean?
Men and non-pregnant women will normally have only small amounts of prolactin in their blood. The levels are ideally interpreted knowing when the sample was collected. The levels will vary over a 24 hour period - rising during sleep and peaking in the morning. Ideally, your blood sample should usually be taken a couple of hours after waking up, preferably after you have been resting quietly for 30 minutes (although your doctor may have reasons for doing them at other times).
High levels of prolactin (hyperprolactinaemia) are normal during pregnancy and after childbirth while the mother is breastfeeding. High levels can also be seen with:
Is there anything else I should know?
Stress from illness, trauma, work and personal problems, and even the fear of having the blood test done can cause moderate increases in prolactin.
Prolactinomas are often small. Your doctor may request an MRI (magnetic resonance imaging) scan to help locate the tumour within the pituitary gland and to look at both the size of the tumour and the size of the pituitary (which often enlarges).
Testosterone (levels will usually be low in a male when prolactin is high), FSH and LH (to help evaluate ovulation and fertility), an MRI scan (to show pituitary enlargement and help locate a tumour), and an eye examination (to evaluate visual disturbances). Further laboratory investigations may be required to ensure the elevated prolactin is not due to macroprolactin.
Increased levels of prolactin are often seen in people with hypothyroidism. If you have hyperprolactinaemia, your doctor will most likely test you for hypothyroidism. Patients with hypothyroidism may have elevated prolactin levels due to the thyroid abnormality, which when treated returns the prolactin to normal.
Prolactinomas are usually treated with medications that act like dopamine (such as bromocriptine or cabergoline). Treatment can reduce prolactin levels and symptoms and restore fertility but the medications may have to be taken for several months or years. Surgery is sometimes necessary if the prolactinomas are large or not responding to treatment. This surgery is delicate and requires an experienced surgeon. Sometimes, despite medication and/or surgery, the prolactinoma will return.
Macroprolactinaemia is a term used to describe the condition where the majority of the circulating prolactin is found to be bound to an antibody. Macroprolactin has limited bioactivity and its presence would be suspected in cases with elevated prolactin levels without the symptoms of prolactin excess. Most laboratories can perform a procedure to remove the antibody bound fraction of prolactin and thus estimate the true active fraction of the circulating prolactin.
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