Blood prolactin (PRL) test and metoclopramide (MTC) stimulation test

Prolactin (PRL)

Prolactin (PRL) is a hormone secreted by the pituitary gland — a gland inside the skull that produces many different hormones. It starts circulating in the blood during puberty, when it is responsible for breast growth in girls. Prolactin levels determine the size and shape of a woman’s breasts. Together with progesterone and oestrogen, it is also involved in the menstrual cycle. Like adrenaline and cortisol, it is also considered a stress hormone. Prolactin is produced in small amounts before menstruation, causing breast tenderness and increased irritability at this time. Increased prolactin secretion, in turn, is associated with pregnancy. This hormone is responsible for milk production (lactation). It is also thought to be a “bonding hormone”, helping to create a close bond between an infant and their mother. Prolactin is said to be closely linked to the emergence of maternal instinct.

It is secreted in both women and men — naturally, its concentration in the blood differs between the sexes.

A prolactin test is performed to establish the cause of unusual nipple discharge, and when a woman has stopped having periods. Difficulty getting pregnant is also an indication for testing. In men, prolactin levels are tested when a problem with the pituitary gland is suspected. Testing is also recommended when a man experiences a loss of sex drive or has erectile dysfunction. Prolactin levels should also be measured if a man has very low testosterone levels. The hormone test should also be performed to check whether a tumour in the pituitary gland is causing the production of large amounts of prolactin.
The test is performed to assess whether prolactin levels are raised (or, very rarely, reduced). Other symptoms of high PRL levels also include

• Infertility resulting from a lack of ovulation during menstrual cycles.
• Seborrhoea, with very oily skin and hair.
• Mild or moderate hirsutism (excessive hair growth on the chin, abdomen or breasts).
• Pain during sexual intercourse.
• Galactorrhoea, meaning milk discharge unrelated to pregnancy or lactation.
• Osteopenia or osteoporosis, meaning reduced bone mineralisation.
• Significant accumulation of fat around the abdomen.
• Substantial weight gain.
• Fatigue, irritability and emotional instability.
• Depressive symptoms and anxiety.
• Frequent headaches.
• Reduced visual acuity and visual disturbances.
• Deficiency or absence of pituitary hormones (hypopituitarism).
• Varying degrees of pituitary overactivity caused by the hormone-secreting activity of a tumour in the hypothalamic–pituitary region, e.g. GH secretion — symptoms of acromegaly, TSH secretion — symptoms of hyperthyroidism, ACTH secretion — symptoms of Cushing’s syndrome.
• Nasal discharge.

The normal prolactin level in women who are not pregnant is considered to be 4-23 nanograms per millilitre (ng/mL) or 4-23 micrograms per litre (mcg/L).

In men, a normal result should fall within the range of 3-15 ng/mL or 3-15 mcg/L. The result report is issued in Polish.

In pregnant women, a normal result is 34-386 ng/mL or 34-386 mcg/L.

A high prolactin level (above 200 ng/mL) may indicate a pituitary gland tumour. The higher the hormone level, the greater the likelihood of disease. If tests show a prolactin level above 200 ng/mL, magnetic resonance imaging is usually performed. Remember that a low or normal hormone level does not rule out a tumour.

Causes of hyperprolactinaemia (excessive prolactin in the blood)

• Taking medicines that increase PRL levels — the most common cause of the disorder. These are most often antidepressants and other medicines acting on the nervous system, as well as medicines that lower blood pressure.
• Other conditions outside the pituitary gland that cause increased prolactin levels — most commonly hypothyroidism.
• Excessive prolactin secretion with no identifiable cause and no abnormalities in the pituitary gland, known as “functional hyperprolactinaemia”
• Prolactin-secreting pituitary adenomas (small tumours), known as prolactinomas
• Other tumours inside the skull that press on the pituitary stalk (the structure connecting the pituitary gland to the rest of the brain).

Metoclopramide (MTC) stimulation test:

If hyperprolactinaemia is suspected, or you already know that your level of this hormone is raised, it is time for diagnostic testing. The most important test for investigating the cause or distinguishing between the conditions responsible for hyperprolactinaemia is the metoclopramide test.

Metoclopramide is an anti-sickness medicine that stimulates intestinal movement. It is used to prevent vomiting and also for constipation. However, because of its mechanism of action (it is a dopamine receptor antagonist and therefore stimulates prolactin secretion), it has also been used in the diagnosis of hyperprolactinaemia. The normal response to metoclopramide is an increase in blood prolactin concentration to a maximum of 6 times the level measured before it was given.

How the metoclopramide (MTC) test is performed

The diagnostic test is carried out in two or three stages. You should attend on an empty stomach (a detailed description of how to prepare for a blood prolactin test appears later in this article). First, at the so-called time point 0, for example at 8 am, a blood sample is collected to measure your prolactin level. Immediately afterwards, you take 1 metoclopramide tablet by mouth. After an hour, at 9 am in our example, blood is collected again to measure the prolactin concentration. If the test has three stages, the blood test must be repeated after another hour, at 10 am. Once the measurements are ready, you receive a prolactin profile, showing the prolactin concentration in your blood before metoclopramide takes effect and at 60 and 120 minutes after taking it.

IMPORTANT! For the MTC test, you must have a referral from a doctor and the prescription medicine metoclopramide, purchased beforehand from a pharmacy.

Preparing for a prolactin test:

The prolactin test result may be affected by strenuous exercise, stress, sleep problems, nipple stimulation, taking certain medicines or cocaine, and having a test involving a radioactive tracer during the week before the prolactin test. If you are planning to have your blood prolactin level tested, you should therefore be reasonably well rested and have had enough sleep. For 24h before the test, you should avoid physical exercise and sexual intercourse, and try to avoid stress. You should usually attend for the test about 3 hours after waking, between 8 and 10 am. People being tested are usually asked to sit still for about half an hour before blood collection. You should, of course, attend for a blood prolactin test on an empty stomach, meaning at least 8h without food and 4h without drinking any fluids. If you are having a metoclopramide stimulation test, make sure the laboratory has this medicine and arrive early in the morning, as you will not be allowed to leave the clinic or collection point, or eat a meal, for an hour or two.

There is no doubt that hyperprolactinaemia is a relatively common disorder, given the many factors that can cause it. For the same reason, identifying its cause and clinical significance is far from straightforward. It is worth checking your blood prolactin level beforehand with a simple test.

* If your results fall outside the normal range, we recommend consulting a doctor.

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