Prolactin has one job most people already know: it tells the breast to make milk. That is true, and it is almost the least interesting thing about it. The reason prolactin shows up on a hormone workup for someone who is not pregnant and not breastfeeding is a different mechanism entirely. In excess, prolactin does not just add a symptom of its own. It quiets the hormones underneath the symptoms it gets blamed for, including the ones covered in earlier pieces in this series on testosterone, estrogen, and the pituitary signals that drive them.
What prolactin actually does
Prolactin is made by the pituitary gland, the same small structure at the base of the brain that also releases FSH and LH. According to Cleveland Clinic, prolactin (also called lactotropin, or PRL) is a hormone responsible for lactation and certain breast tissue development, and it contributes to hundreds of other bodily processes beyond that. During pregnancy, it works alongside estrogen and progesterone to stimulate breast tissue development and milk production.
What makes prolactin unusual among hormones is how it is controlled. Most hormones answer to a releasing signal that turns them up. Prolactin mostly answers to a brake. Cleveland Clinic notes that dopamine, a brain chemical, along with estrogen, controls prolactin production, and dopamine's role here is inhibitory: it keeps the pituitary's output turned down by default. Loosen that brake, whether through a medication, a tumor, or a disrupted signal from the hypothalamus, and prolactin climbs.
Prolactin also has a rare feature: Cleveland Clinic describes it as one of the few hormones with a positive feedback loop, meaning its own release stimulates the pituitary to release even more of it, and nipple stimulation from breastfeeding pushes that loop further. That mechanism is exactly what nursing depends on. Outside of that context, a prolactin level that stays high on its own is worth understanding rather than shrugging off.
Why it gets tested outside of pregnancy
A prolactin test is not part of a routine annual panel. It gets ordered when specific symptoms point toward it. MedlinePlus lists what those look like by sex. In women, the symptoms include changes in menstruation, irregular or absent periods, unexpected breast milk production, nipple discharge, and infertility. In men, they include nipple discharge, erectile dysfunction, enlarged breast tissue, decreased facial and body hair, and low sex drive.
Those symptoms are not random. They trace back to a single mechanism: prolactin, at high levels, interferes with the hormones that run the reproductive cycle in both sexes.
The suppression mechanism
This is the piece that connects prolactin to the earlier posts in this series on FSH, LH, and testosterone. High prolactin does not act alone on the ovaries or testes. It acts upstream, on the signal that starts the whole cascade. Testing.com describes the chain plainly: high prolactin suppresses gonadotropin-releasing hormone, which in turn drives LH and FSH, and without those two firing normally, ovulation stops and periods turn irregular or disappear. The same suppression lowers testosterone in men, which is why low libido and erectile dysfunction sit on the symptom list alongside the more visibly reproductive ones.
That is a useful reframe. A hormone panel that comes back with low LH, low FSH, and low testosterone or estrogen does not always point to a problem with the ovaries, testes, or the pituitary's output of those specific hormones. Sometimes the root cause sits one level up: prolactin turned up, quietly turning everything downstream of it down. That is also why prolactin is one of the first things a clinician checks when working up unexplained infertility, or a low testosterone result that does not fit the rest of the picture.
What counts as high, and what causes it
Reference ranges vary by lab, but Cleveland Clinic gives a general anchor: normal prolactin runs below 20 ng/mL for men, below 25 ng/mL for women who are not pregnant or breastfeeding, and 80 to 400 ng/mL for those who are pregnant or breastfeeding. Testing.com's guide breaks the elevated range into rough tiers, from mild elevation up through markedly high results, while cautioning that these are lab-dependent reference points, not fixed cutoffs everyone shares.
The most common cause of a persistently high result is a prolactinoma, a benign pituitary tumor that itself produces prolactin. MedlinePlus lists it first among causes, describing it as a noncancerous growth on the pituitary gland that manufactures prolactin directly. Other drivers on MedlinePlus's list include certain medicines, hypothyroidism, kidney disease, chest injuries, and other pituitary tumors. Testing.com names the frequent medication offenders specifically: antipsychotics, certain antidepressants, and drugs that affect dopamine, with metoclopramide and verapamil called out by name.
Then there is the everyday noise. A single elevated number does not mean much on its own. MedlinePlus points out that small, temporary increases can come from stress, exercise, sex, or even a high-protein meal, which is why a provider will often simply retest before reading too much into one result. That is also why timing matters more here than with most blood draws: a sample is usually scheduled three to four hours after waking, with overnight fasting and calm beforehand, to get a stable read.
The macroprolactin wrinkle
There is a specific lab nuance worth knowing if a result comes back high without matching symptoms. Some people carry a form called macroprolactin, which the clinical reference StatPearls describes as a large circulating aggregate of prolactin bound to antibodies. It measures as prolactin on a standard test but carries little or no biological activity, and mistaking it for true excess prolactin can lead to an unnecessary diagnosis. Labs can screen for it with a polyethylene glycol, or PEG, precipitation step that filters the inactive aggregate out before reporting a final number.
Where this leaves the reading
Prolactin is not a hormone to fear or chase. For most people who are not pregnant or breastfeeding, it should sit quietly in the background. When it does not, the pattern of symptoms, not the number alone, tells the story: irregular cycles, unexpected milk production, a drop in libido, or a testosterone result that will not budge despite an otherwise normal picture upstream. Read alongside LH, FSH, and thyroid-stimulating hormone, a prolactin result completes a piece of the pituitary story that none of those markers can tell on their own.
Common questions
Can stress alone raise prolactin enough to cause symptoms?
Stress can raise prolactin, but usually only temporarily. A single elevated reading is typically followed by a repeat test rather than an immediate diagnosis, since exercise, sex, and even a recent meal can do the same thing. Sustained symptoms alongside a repeatedly high result point toward a more lasting cause, such as a medication, a thyroid problem, or a pituitary tumor, rather than an ordinary stressful week.
Is a high prolactin result always a sign of a tumor?
No. A prolactinoma is the most common cause of a persistently high result, but MedlinePlus and Testing.com both list several other explanations, including hypothyroidism, kidney disease, certain medications such as antipsychotics and metoclopramide, and even a biologically inactive form of prolactin called macroprolactin that can inflate a standard test without any underlying disease. Imaging is generally reserved for cases where a repeat blood test confirms the elevation and the symptoms fit.
Why would a doctor check prolactin when investigating low testosterone?
Because prolactin acts upstream of testosterone, not on it directly. High prolactin suppresses the release of gonadotropin-releasing hormone, which in turn lowers LH and FSH, the two pituitary signals that tell the testes to produce testosterone. A low testosterone result paired with low LH and FSH is one of the situations where checking prolactin can reveal the actual starting point of the problem, rather than treating the testosterone number in isolation.
Do men need to worry about prolactin, or is it mainly a women's hormone?
Both sexes produce prolactin and can develop hyperprolactinemia. In men, high prolactin can cause low sex drive, erectile dysfunction, enlarged breast tissue, and reduced facial and body hair, largely through the same testosterone-suppressing pathway that disrupts women's cycles. It gets tested less often in men simply because the resulting symptoms are less immediately obvious than a missed period.
What is the right way to prepare for a prolactin test?
Morning testing, roughly three to four hours after waking, gives the most stable reading, since prolactin follows its own rhythm through the day. Overnight fasting is generally preferred, and it helps to avoid strenuous exercise, sex, and stressful situations for several hours beforehand, since all three can temporarily push the number higher.
