Prolactin is a pituitary hormone that stimulates breast milk production, and high levels can mimic PCOS.
What prolactin does
Prolactin is a hormone released by the pituitary gland in the brain, a small gland that controls many other hormones in the body, and its main job is to stimulate milk production during breastfeeding. Everyone has prolactin at a certain baseline level, not only while breastfeeding. The level can also rise briefly, for example after sleep, exercise, or during stress. For this reason, a single high reading does not necessarily mean there is a problem. When levels stay high, the situation is called hyperprolactinaemia, and it can disturb the hormonal balance that controls the menstrual cycle. The hormonal background is explained on the hormones page.
Why it is tested when investigating PCOS
When prolactin is high, it can suppress ovulation and cause infrequent or absent periods. These features are also seen in PCOS, so the two can look similar at first. For that reason, doctors usually test prolactin when investigating irregular periods, to rule out other causes before drawing any conclusion about PCOS. This test is often done together with others, such as a thyroid check, so the overall picture is more complete. This step matters so that any treatment truly matches the real cause. More about the condition is on the what is PCOS page.
What can cause high levels
High prolactin can come from certain medicines, stress, an underactive thyroid, or a small, usually harmless pituitary tumour. Because the causes are varied, a single test result is not enough on its own. The result is interpreted in context and sometimes repeated for confirmation, since the level can rise briefly for ordinary reasons.
When to talk to a doctor
If your periods are irregular, a doctor can assess whether a prolactin test suits you and explain what the result means. They can also check whether any medicines you take might affect the result, since some medicines can raise prolactin. It is best not to interpret a single number on your own without advice. Any decision about further testing or treatment should be discussed with a doctor or pharmacist who knows your health history.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.