Serial prolactin

Serial prolactin is a test in which prolactin is measured three times over two hours in the morning: initially and after 60 and 120 minutes. It shows whether prolactin remains raised throughout the procedure or has risen only temporarily, most often because of stress or exertion before blood collection. A doctor usually requests it when a single measurement of prolactin was mildly raised or the result does not match the symptoms.

Who the test is for

• people whose prolactin was mildly raised in a single measurement, when a doctor wants to exclude a temporary rise;
• women with irregular or absent periods and people undergoing fertility investigation when previous results are unclear;
• people with milky breast discharge outside pregnancy and breastfeeding;
• men with reduced libido or erectile dysfunction and borderline prolactin.

What prolactin is and why it is measured repeatedly

Prolactin is secreted by the anterior pituitary. After childbirth it stimulates milk production, while outside pregnancy and breastfeeding a persistently raised level inhibits the hormones controlling ovarian and testicular function. Women may then have absent periods and ovulation, and may develop milky breast discharge. In men, raised prolactin can lower testosterone and libido and cause erectile dysfunction.

Prolactin is secreted in bursts, rises during sleep, and is temporarily increased by stress, physical exertion and breast stimulation. Even the discomfort of blood collection can raise it in some people. One raised result therefore does not necessarily mean disease. Measuring prolactin three times over two hours shows whether it falls towards normal values or remains raised. A doctor decides whether you need one measurement or serial prolactin.

Preparation and sample

Prolactin is measured in venous blood (serum), with blood collected three times: initially, after 60 and after 120 minutes. Allow a little over two hours for the test. Prepare as follows:

• come in the morning, with the first collection no earlier than 2 hours after waking;
• come rested and without rushing;
• avoid strenuous exercise, sexual intercourse and breast stimulation the day before and on the collection day, and schedule any breast examination or procedure after testing;
• fasting is not required, but avoid a large meal before collection;
• do not stop medicines yourself, but state which you take; if you take biotin (for example hair and nail supplements), mention this during blood collection.

Blood is taken on three occasions over two hours, during which you remain at rest.

How to interpret the result

The report contains all three values. The reference range depends on sex and method, with physiologically higher levels during pregnancy and breastfeeding; the range is printed on the report. A doctor interprets results alongside symptoms, medicines and other hormones.

• All three values within range: prolactin is not raised at the time of testing.
• First value raised, subsequent values lower and within range: the rise was most likely temporary, for example due to stress.
• All three values raised: prolactin remained raised throughout the procedure. A doctor assesses the result alongside medicines, physiological causes and macroprolactin, confirms it further if necessary and investigates the cause.

Causes of persistently raised prolactin

• pregnancy and breastfeeding, when a rise is normal;
• medicines that inhibit dopamine action (dopamine antagonists): antipsychotics such as risperidone or haloperidol, and the anti-nausea medicines metoclopramide and domperidone; some antidepressants, oestrogens and opioids can also raise prolactin;
• an underactive thyroid, so prolactin is measured alongside TSH;
• chronic kidney or liver disease;
• prolactinoma, a benign prolactin-secreting pituitary tumour, and other changes in the pituitary region, demonstrated by magnetic resonance imaging.

A raised result without symptoms may be due to macroprolactin, prolactin bound to antibodies in larger complexes. It raises the measured result but has almost no activity in the body and requires no treatment. When the result and symptoms do not match, a doctor may request an additional macroprolactin test.

Low prolactin

Low prolactin is rarely significant. It most often results from prolactin-lowering medicines and rarely from reduced pituitary function. After childbirth, it can make milk production difficult.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Come in the morning. Prices for all tests and panels are in the service price list. If a doctor requests only one measurement, a prolactin test is sufficient; it is also part of the Fertility investigation panel.

Frequently asked questions

What is the difference between prolactin and serial prolactin?

For prolactin, blood is collected once. For serial prolactin, it is collected three times over two hours, showing whether a rise is temporary or persists throughout the procedure. Serial prolactin usually follows a single mildly raised result.

Can stress raise prolactin?

Yes. Stress, exertion, breast stimulation and discomfort during blood collection temporarily raise it. This is precisely why serial prolactin monitors how the value changes over two hours.

What if all three values are raised?

Prolactin has then remained raised throughout the procedure. A doctor assesses the result alongside medicines, pregnancy and other physiological causes and macroprolactin, and confirms it further if necessary. The cause is also investigated by checking thyroid and kidney function; very high values, headaches or visual disturbances lead to referral for pituitary magnetic resonance imaging. Prolactinomas are almost always benign and usually respond well to medication.

Do medicines affect the result?

Many medicines raise prolactin, particularly antipsychotics and anti-nausea medicines, while medicines such as cabergoline lower it. Do not stop them yourself; tell the doctor interpreting your result about them.