Biomarker 101: Prolactin

Prolactin is best known for milk production, but it turns up on hormone panels for reasons that have little to do with breastfeeding — and it is unusually easy to raise by accident.

What prolactin is

Prolactin is produced by the pituitary gland at the base of the brain. Its most familiar role is stimulating milk production after childbirth, but it also influences the menstrual cycle, fertility, libido and immune function in both sexes.

Why it's measured

Prolactin is measured when investigating irregular or absent periods, difficulty conceiving, low sperm count, reduced libido or erectile difficulties. It is also measured when a pituitary cause is suspected, and to monitor a known condition over time.

It sits on hormone panels because raised prolactin suppresses other reproductive hormones. If someone has unexplained cycle problems or low testosterone, prolactin is one of the things worth ruling in or out.

What the numbers mean

Reference ranges differ by sex and by laboratory, and are naturally much higher in pregnancy and while breastfeeding.

Mild elevations are common and frequently mean nothing. Many people with slightly raised prolactin have no symptoms at all, and a modest result is usually repeated after a few weeks before anything is concluded.

What raises prolactin

This is where prolactin differs from most markers. A number of ordinary things raise it temporarily:

• Stress, including the stress of the blood draw itself
• Sleep — levels are higher shortly after waking
• Exercise before the test
• Recent sexual activity or nipple stimulation
• Eating shortly beforehand

Beyond those, several medication classes raise prolactin as a known effect, and it also rises with an underactive thyroid, with polycystic ovary syndrome, with reduced kidney or liver function, and with a benign pituitary growth that produces prolactin directly.

Because so many benign factors influence it, the collection conditions matter. A result taken after a rushed, stressful morning is not the same as one taken calmly.

What raised prolactin can cause

Where levels are persistently high, the effects follow from prolactin suppressing other reproductive hormones. In women that can mean irregular or absent periods and difficulty conceiving. In men it can mean reduced libido and fertility problems. Milky nipple discharge unrelated to breastfeeding can occur in either sex.

What prolactin won't tell you

• One raised result proves nothing. Repeat testing under better conditions is the standard next step.
• It doesn't identify the cause. Distinguishing a medication effect from a thyroid cause from a pituitary one requires further investigation.
• Mild elevations are usually benign. The degree matters, and small increases are common.
• Never stop a medication over a result. If a medication may be involved, that is a conversation with the prescribing doctor.

The bottom line

Prolactin is genuinely informative when investigating reproductive symptoms, and genuinely easy to raise through ordinary circumstances. Its main practical lesson is that how a sample is taken can matter as much as what it shows.