Prolactin Blood Test UK: The Hormone That Can Quietly Switch Off Testosterone

What prolactin actually does

Prolactin is made by the anterior pituitary, the small gland at the base of the brain that conducts much of the hormone orchestra. Its headline job is lactation: after childbirth, prolactin is what drives the breasts to produce milk. That is why the name tends to read as a women's hormone, and why its role in men gets overlooked.

Both sexes carry prolactin all the time, usually at a low baseline. It is held in check by dopamine, which the brain releases to keep prolactin down. Anything that interferes with that dopamine signal, or that stimulates the pituitary directly, lets prolactin rise. The consequences depend on how high it goes and why.

The reason prolactin earns a place on a hormone panel is what it does when it climbs. A high level suppresses the release of gonadotrophin- releasing hormone from the brain, which in turn lowers LH and FSH, the two signals that tell the testes and ovaries to make sex hormones. The result is a fall in testosterone in men and disrupted ovulation in women, traced back to a hormone most people have never heard of.

High prolactin in men: when testosterone looks fine but feels low

In men, a raised prolactin is one of the genuinely treatable causes of low testosterone, which is exactly why it should be measured before anyone reaches for testosterone replacement. The mechanism is simple: high prolactin dampens the brain's signal to the testes, LH and FSH fall, and testosterone production drops with them.

The symptoms are the familiar picture of low testosterone, with a few tells of their own. Low libido and erectile difficulty are common, and often more prominent than they would be for a comparable drop in testosterone from another cause. Fatigue, low mood and loss of muscle follow. Some men develop breast tenderness or enlargement, and a small number notice milk-like nipple discharge, an unmistakable prompt to check prolactin. Fertility suffers because LH and FSH drive sperm production as well as testosterone.

This is the scenario where a lone testosterone test misleads. A man can sit at the lower end of the testosterone range, feel every symptom, and never learn why until prolactin, LH and FSH are read together. That interpretation, low testosterone with low LH and FSH and a high prolactin, is the pattern a qualified clinician looks for, and it points somewhere completely different from age-related decline.

What causes high prolactin

Prolactin rises for reasons that range from entirely benign to worth investigating, which is why the size of the rise matters as much as its presence. The most important pathological cause is a prolactinoma, a benign tumour of the pituitary gland that secretes prolactin. These are common, almost always non-cancerous, and very treatable, but a markedly high prolactin is the finding that prompts a clinician to look for one, usually with a pituitary MRI.

Medication is the everyday cause people forget. Many antipsychotics raise prolactin sharply, as do some antidepressants, the anti-sickness drug metoclopramide, and opioids. An underactive thyroid lifts it too, because the body's response to low thyroid hormone stimulates prolactin release, which is one reason thyroid markers belong on the same panel. Chronic kidney disease, chest wall injury and even persistent nipple stimulation can nudge it up.

Then there are the harmless spikes that catch out a poorly timed test. Stress, a recent meal, vigorous exercise and disturbed sleep all raise prolactin transiently. So does a stressful blood draw itself. This is why a single mildly raised result is rarely the end of the story, and why how you take the test, covered below, changes whether the number means anything.

High prolactin in women

Outside pregnancy and breastfeeding, where high prolactin is normal and expected, a raised level in women disrupts the menstrual cycle. The same suppression of LH and FSH that lowers testosterone in men interferes with ovulation, so the typical presentation is irregular or absent periods, difficulty conceiving, and sometimes milk-like discharge from the breasts when not breastfeeding.

Because those symptoms overlap with other hormone problems, prolactin is a standard part of investigating irregular cycles and fertility concerns, sitting alongside the markers covered in our fertility blood test guide and our guide to hormone imbalance testing. A mildly raised prolactin can also appear in PCOS, which is why the full hormone picture, not a single marker, is what a clinician reads.

How to take a prolactin test so the result is real

Few hormones are as sensitive to how the sample is taken as prolactin. It is highest during sleep and in the first hours after waking, and it jumps with stress, food and exertion. A test taken at the wrong moment can return a falsely high number that sends an otherwise well person down a path of unnecessary worry and scanning.

The practical advice is to test mid-morning, ideally three to four hours after waking, rested, without strenuous exercise beforehand and without nipple stimulation in the hours before the draw. Sitting quietly for a few minutes before the sample helps, because the stress of the needle itself can lift the reading. If timing matters for the markers you are checking, our guide to when to take a hormone blood test walks through the windows that matter.

One more quirk is worth knowing. Some people carry macroprolactin, a biologically inactive form of the hormone that the lab assay still counts. It can produce a high reading in someone with no symptoms at all. A good laboratory screens for it when prolactin comes back raised, which stops a harmless quirk being mistaken for a problem.

Reference ranges, and why the number alone is not a diagnosis

UK laboratories usually report prolactin in milli-international units per litre (mIU/L). Ranges vary by assay, but a common adult reference is roughly 86 to 324 mIU/L for men and 102 to 496 mIU/L for non-pregnant women. Some labs quote micrograms per litre (µg/L) instead, where the figures are far smaller. The range printed on your own report always takes precedence, because it reflects the exact method your lab used.

What the number means depends entirely on its size and company. A slightly raised result, especially from a poorly timed or stressful draw, often resolves on a repeat test taken properly. A modestly high level prompts a look at medication and thyroid. A markedly high level is the finding that leads a clinician to investigate the pituitary directly. None of those conclusions can be drawn from the prolactin figure on its own, which is the whole argument for reading it inside a panel.

What a low prolactin means

A low prolactin rarely causes problems and is seldom the reason a test is ordered. The exception is when the whole pituitary is underperforming, a state called hypopituitarism, where prolactin sits low alongside other pituitary hormones. In that setting it is the pattern across several hormones, not the prolactin alone, that tells the story, and it is uncommon. For most people, a low or low-normal prolactin is simply unremarkable.

NHS testing versus a full hormone panel

A GP can and does order prolactin, typically when investigating low testosterone in men, irregular periods or infertility in women, or symptoms such as nipple discharge. It is not part of routine screening, so a well person curious about their hormone picture is unlikely to be offered it without a specific prompt.

The value of testing privately is reading prolactin in the company that makes it meaningful. Seen next to total and free testosterone, LH, FSH and thyroid markers, it stops being a lonely figure and becomes the piece that explains a result the headline testosterone could not. Helvy's Complete Male Hormones panel measures prolactin with that full set, so if a raised level is dragging your testosterone down, the pattern is visible in one test rather than pieced together over several. For the wider context on which markers belong together, our guide to low testosterone symptoms in men sets out the full workup.

Frequently asked questions