Prolactin

What is being tested?
Prolactin is a hormone produced by the front part (anterior lobe) of the pituitary gland, a grape-sized organ located at the base of the brain. Prolactin secretion is regulated and inhibited by dopamine produced by the brain. Prolactin is normally present in small amounts in men and in women who are not pregnant, and its main role is to promote lactation (breast milk production).
Prolactin levels are generally high throughout pregnancy and immediately after childbirth. During pregnancy, hormones such as prolactin, oestrogen and progesterone stimulate breast milk production. After childbirth, prolactin regulates the start and maintenance of breast milk production. If a woman does not breastfeed, her prolactin level soon falls back to pre-pregnancy levels. If she does, nursing the baby plays an important role in releasing prolactin. There is a regulatory mechanism linking how often the baby is breastfed, the amount of prolactin released by the pituitary gland and the amount of milk produced.

Another frequent cause of raised prolactin levels is a prolactinoma, a prolactin-secreting tumour of the pituitary gland. Prolactinomas are the most common type of pituitary tumour and are usually benign. They occur more often in women but are also found in men. The resulting problems may stem both from the effects of excess prolactin, such as milk production in a woman who is not pregnant (and rarely in a man), and from the size and location of the tumour.

If the anterior pituitary and/or the tumour grow significantly, they can press on the optic nerve, causing headaches and visual disturbances, and can interfere with the other hormones the pituitary gland produces. In women, prolactinomas can cause infertility and menstrual irregularities; in men, these tumours can cause a gradual loss of sexual function and libido. If left untreated, prolactinomas can eventually damage the surrounding tissues.

How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed, but the sample is usually collected 3 to 4 hours after waking; a fasting sample is preferable.

1. How is it used?
2. When is it requested?
3. What does the test result mean?
4. Is there anything else I should know?

1. How is it used?
Prolactin levels are used, together with other hormone tests, to help:

• Determine the cause of galactorrhoea
• Determine the cause of headaches and visual disturbances
• Diagnose infertility and erectile dysfunction in men
• Diagnose infertility in women
• Diagnose prolactinomas
• Assess anterior pituitary function (together with other hormones)
• Monitor the treatment of prolactinomas and detect recurrences

2. When is it requested?
Prolactin levels may be requested when a person has symptoms of a prolactinoma, such as unexplained headaches, visual impairment and/or galactorrhoea. The test may be requested, together with others, when a woman is unable to become pregnant or has irregular periods, or when a man has symptoms such as reduced libido, galactorrhoea or infertility. In men, prolactin testing is often ordered as a follow-up to a low testosterone level.
When a person has a prolactinoma, blood prolactin levels may be measured periodically to monitor the tumour’s progress and its response to treatment. The test may also be used at regular intervals to monitor for recurrence of a prolactinoma.
Prolactin testing may be requested, together with tests for other hormones such as growth hormone, when the doctor suspects that a person has more general hypopituitarism. Prolactin concentrations may sometimes be monitored
when a person is taking medicines or has a condition that affects dopamine production.

3. What does the test result mean?
Serum prolactin is normally low in men and in women who are not pregnant. Prolactin levels must, however, be assessed in relation to the time of day at which the sample is collected. Levels vary over 24 hours, rising during sleep and peaking in the early morning. A person’s blood sample should usually be taken 3 to 4 hours after waking, preferably after the person has been resting quietly for 30 minutes, although the doctor may have reasons for taking the sample at other times.
High prolactin levels (hyperprolactinaemia) are normal during pregnancy and after childbirth while the mother is breastfeeding. High levels are also seen with:

• Anorexia nervosa
• Medicines: oestrogens, tricyclic antidepressants, opiates, amphetamines, blood pressure medicines (reserpine, verapamil, methyldopa) and some medicines used to treat gastro-oesophageal reflux (cimetidine)
• Hypothalamic diseases
• Hypothyroidism
• Kidney disease
• Nipple stimulation (moderate increase)
• Other pituitary tumours and diseases
• Polycystic ovary syndrome
• Prolactinomas

Lower than normal prolactin levels are usually not treated, but may indicate more general hypopituitarism. Low levels may also be caused by medicines such as dopamine, levodopa and ergot alkaloid derivatives.

4. Is there anything else I should know?
Stress from illness, trauma and even anxiety about having a blood test can cause moderate increases in prolactin.
Prolactinomas are often small. Together with prolactin testing, a doctor may order an MRI (magnetic resonance imaging) scan of the brain to locate the tumour and determine its size and the size of the pituitary gland.

1. What other tests may be done to investigate raised prolactin?
2. If my prolactin is raised, why is my doctor ordering thyroid tests?
3. How are prolactinomas treated?
4. What is macroprolactin?

1. What other tests may be done to investigate raised prolactin?
Other tests may be done, such as testosterone (the level is usually low in a man when prolactin is high), FSH and LH (to help assess ovulation and fertility), an MRI scan of the brain (to show enlargement of the pituitary gland and help locate a tumour) and an eye examination (to assess visual disturbances). Raised prolactin levels require laboratory tests to make sure the increase is not due to macroprolactin (see question 4).

2. If my prolactin is raised, why is my doctor ordering thyroid tests?
Raised prolactin levels are often seen in people with hypothyroidism (although it is not the cause). If you have hyperprolactinaemia, your doctor will very probably order tests to check your thyroid function.

3. How are prolactinomas treated?
Prolactinomas are usually treated with medicines that act as dopamine agonists (e.g. bromocriptine or cabergoline). Treatment can reduce prolactin levels and symptoms and restore fertility, but the medicines may need to be taken for several months or years. Surgery is sometimes needed if prolactinomas are large or do not respond to treatment. This operation is delicate and requires an experienced surgeon. Sometimes, despite medication and/or surgery, the prolactinoma comes back.

4. What is macroprolactin?
Some healthy people have high prolactin levels because most of the prolactin in their blood is in a different form called macroprolactin. Macroprolactin does not come from the pituitary gland, is not active in the body and does not indicate the presence of disease. If raised prolactin is found, further laboratory tests must be carried out to make sure this is not due to the presence of macroprolactin.