A prolactin blood test measures prolactin, a pituitary hormone that drives milk production after childbirth and, when it runs high at other times, switches off the reproductive system in women and men. Clinicians order it for missed or irregular periods, infertility, milky nipple discharge, low libido, and erectile dysfunction, and to check for a benign pituitary growth called a prolactinoma. It is a single morning draw with no fasting. Mildly high results are common and often explained by stress or medication, so one is usually repeated first.

What prolactin does

Prolactin is made by the pituitary, the gland at the base of the brain, and in a nursing mother it is what makes milk. Outside pregnancy and breastfeeding, it is kept low by dopamine. When it rises anyway, it suppresses the signals that drive ovulation and testosterone production. That is why high prolactin shows up as absent periods and infertility in women, and as low libido, erectile dysfunction, and low testosterone in men, long before anyone thinks of the pituitary.

Levels follow a daily rhythm, highest during sleep and early morning, and they spike briefly with stress, exercise, eating, sex, and nipple stimulation. Each of those can turn a normal result into a mildly high one if the draw is badly timed.

Who needs a prolactin blood test

In women, the common reasons are periods that have stopped or become infrequent, difficulty conceiving, and milky discharge from the nipples without pregnancy. Prolactin is also part of the workup for polycystic ovary syndrome, because the two conditions overlap in symptoms and need different treatment, and a prolactin blood test belongs on any fertility panel alongside FSH, LH, estradiol, and TSH.

In men, the reasons are low libido, erectile dysfunction, infertility, and any finding of low testosterone, since high prolactin is one of its correctable causes. Breast tissue growth is a less common trigger.

In either sex, new headaches or changes in vision raise the question of a pituitary growth pressing on nearby structures, and people on medications known to raise prolactin, including many antipsychotics and some antidepressants, are tested when symptoms appear. Anyone already treated for a prolactinoma has the test on a schedule to monitor the response.

How to prepare

Fasting is not required, though a meal can nudge prolactin up slightly, so a morning draw before breakfast is tidy. What matters more is being rested. Draw in the morning at least an hour after waking, after sitting quietly for a while, and avoid intense exercise, sex, and nipple stimulation for 24 hours before. A stressful or difficult draw itself raises prolactin, which is one reason a mildly high first result is often repeated.

List every medication and supplement. Prolactin is raised by antipsychotics, some antidepressants, metoclopramide and similar anti-nausea drugs, some blood pressure medicines, and estrogens, and a result cannot be interpreted without knowing what you take. Pause high-dose biotin for two days, since it interferes with many hormone assays.

Reading your result

Reference ranges vary by lab and are different for women and men; your report shows the range that applies. What a clinician does next depends on how high the value is, whether symptoms match, and what else is going on.

A mildly high result, up to about twice the upper limit, is the most common abnormal finding and often reflects stress, a stressful draw, recent exercise, or medication. The Endocrine Society guideline is clear that a single measurement above the lab's range, taken without excessive stress, is enough to diagnose hyperprolactinemia and that dynamic tests of prolactin secretion add nothing. In practice, a calm morning repeat is the usual next step. If it stays high, the clinician checks TSH, because an underactive thyroid raises prolactin, runs a pregnancy test where relevant, and reviews medications.

Two laboratory quirks matter here. Macroprolactin is a large, inactive form of prolactin that some assays count, inflating the number in people with no symptoms; the guideline suggests checking for it when prolactin is high without a matching clinical picture, and some labs do this automatically, as Mayo Clinic's test catalog notes. The hook effect is the opposite problem: a very large prolactinoma can produce so much prolactin that the assay reads falsely low, and labs resolve it by diluting the sample.

A markedly high result, several times the upper limit and especially above about 250 ng/mL, points toward a prolactinoma, and pituitary MRI follows along with other pituitary hormone tests. Prolactinomas are benign and are the most common pituitary tumor. Most respond to a tablet, usually cabergoline, that lowers prolactin, shrinks the growth, and restores periods, fertility, and libido. Our guide to high prolactin goes through causes and next steps in more detail.

A low result is rarely significant on its own and needs no action unless other pituitary hormones are also low.

What the test cannot tell you

A prolactin blood test cannot distinguish a prolactinoma from a medication effect or a thyroid cause on its own; the level is a strong hint, but the history and the follow-up tests make the diagnosis. It also cannot see the pituitary, which is what imaging is for. And in pregnancy and breastfeeding, prolactin is high by design, so testing is not routinely useful.

How to get tested

You can order one yourself. August's lab tests include a prolactin test on its own and within the women's and men's hormone panels, at one price that covers the draw. A phlebotomist comes to your home for a rested morning draw, which is the ideal condition for this hormone, or you visit a nearby lab. Results post the next day in most cases, and a clinician reviews every one before you see it, so a mildly high value comes with the context that it may need a calm repeat rather than alarm. August Care members get up to 50 percent off next-day lab tests and a first consultation included.

Not sure whether your symptoms point to prolactin or another hormone? Describe them to August, a free AI health assistant, to help decide what to test. It is a starting point, not a diagnosis.

Frequently Asked Questions

The level of prolactin, a pituitary hormone that drives milk production and, when elevated at other times, suppresses ovulation and testosterone. It is ordered for irregular or absent periods, infertility, nipple discharge, low libido, erectile dysfunction, and suspected pituitary growths, and to monitor treatment of a prolactinoma.

Fasting is not required, though a morning draw before breakfast is reasonable since meals nudge prolactin slightly. Being rested matters more: draw at least an hour after waking, after sitting quietly, and avoid intense exercise, sex, and nipple stimulation for 24 hours beforehand, because all of them raise the level briefly.

Ranges vary by lab and differ between women and men, so the range printed on your report is the one to use. Pregnancy and breastfeeding raise prolactin by design. A value modestly above the range is common and often reflects stress or medication; values several times the upper limit are more likely to reflect a prolactinoma.

Pregnancy and breastfeeding; stress, including the stress of a blood draw; many medications, especially antipsychotics, some antidepressants, and anti-nausea drugs; an underactive thyroid; prolactinoma and other pituitary conditions; kidney or liver disease; and macroprolactin, an inactive form that inflates the result without causing symptoms.

Not immediately. Mild elevations are the most common abnormal finding and are frequently temporary. The standard next step is a repeat on a calm morning, then a thyroid check, a pregnancy test where relevant, and a medication review. Persistent elevation without an explanation is what leads to pituitary imaging.

A benign growth in the pituitary gland that produces prolactin. It is the most common pituitary tumor and is not cancer. Most are treated with a tablet that lowers prolactin and shrinks the growth, restoring periods, fertility, and libido. Large ones can cause headaches or affect vision and need prompt care.

Yes, in two directions. Macroprolactin, an inactive form, can make the number read high in someone with no symptoms, and labs can screen for it. The hook effect can make a very large prolactinoma read falsely low, which labs resolve by diluting the sample. A clinician who knows your symptoms will recognize both.

Yes. An August phlebotomist draws the sample at home in the morning, which suits this hormone well, since a rested draw in familiar surroundings avoids the stress-related rise seen in clinics. A certified lab runs the test, and a clinician reviews the result before you see it.