Prolactin: what the test measures and what a high result can mean.

Prolactin is the pituitary hormone that drives milk production after a baby. Outside pregnancy and breastfeeding it should sit low, and when it runs high it can quietly switch off ovulation, which is why doctors check it for irregular or missing periods and when someone is trying to conceive. A mildly high result is common and is usually repeated before anyone draws conclusions.

What prolactin is and does

Prolactin is made by the pituitary gland, the same small gland at the base of the brain that releases FSH and LH. Its best-known job is in the name: it prepares the breasts for lactation during pregnancy and keeps milk supply going afterwards. Levels climb steadily through pregnancy and stay raised for as long as you are breastfeeding.

Prolactin also has a second role that matters for your cycle. It dampens the brain's signal to the ovaries. That is useful after a birth, when it helps hold off ovulation while a baby is feeding often. It is less useful if your prolactin is raised for some other reason, because the same dampening effect can make cycles long, irregular or stop them altogether.

Prolactin is released in pulses, rises during sleep and after meals, and responds to stress. Those everyday swings are why the timing of the blood test matters more than for most hormones.

When prolactin is tested

Prolactin usually appears as part of a hormone work-up when a doctor is trying to understand one of the following:

• Irregular or infrequent periods, especially cycles that have stretched well past 35 days without an obvious reason
• Periods that have stopped for three months or more when you are not pregnant, breastfeeding or on hormonal contraception
• Difficulty conceiving, because a raised prolactin can interfere with ovulation even when periods look roughly regular
• Milky nipple discharge when you are not pregnant or breastfeeding
• Low libido or vaginal dryness with no other clear cause
• Headaches or visual changes alongside any of the above, which a doctor will want to look into promptly
• A PCOS assessment, where prolactin is checked so that a similar-looking cause is not missed

Because the same symptoms can come from thyroid problems, low FSH and LH, or PCOS, prolactin is almost always read alongside TSH, FSH, LH and oestradiol. The irregular periods blood tests article explains how those pieces fit together.

Reference range and what pushes it up

Australian laboratories report prolactin in milli-international units per litre (mIU/L). In a woman who is not pregnant or breastfeeding, a result under about 500 mIU/L is usually considered within range, though the exact upper limit differs between labs and assays.

Result	Typical adult range	What it usually means
Within range	Under about 500 mIU/L (non-pregnant, not breastfeeding)	Prolactin is unlikely to be the reason for cycle changes. Your doctor looks at the other hormones.
Mildly raised	Roughly 500 to 1,000 mIU/L	Very often explained by stress, sleep, timing or a medicine. Usually repeated under calmer conditions before anything else is done.
Clearly raised	Well above 1,000 mIU/L	Less likely to be a one-off. Your doctor looks for a cause and may arrange further tests or imaging.
Pregnancy and breastfeeding	Several times the non-pregnant range	Expected. A raised prolactin here is normal physiology, not a problem.

Ranges vary by laboratory; your report shows the range your lab uses. Typical ranges on this page are drawn from Pathology Tests Explained (RCPA) and the sources listed below.

Prolactin is easy to push up by accident. Common, harmless reasons for a raised reading include:

• Stress, including the stress of the blood collection itself, particularly if you find needles difficult
• Recent sleep or a very early collection, since prolactin peaks during the night and takes an hour or two after waking to settle
• A recent meal, vigorous exercise or sexual activity shortly before the test
• Nipple stimulation or chest-wall irritation, including from a tight bra or recent piercing
• Some medicines, which your doctor will ask about; several common prescription medicines raise prolactin as a side effect, and the reading settles once that is accounted for
• An under-active thyroid, which is one reason TSH is checked alongside prolactin
• Reduced kidney function, because prolactin is cleared more slowly, which is why kidney function is sometimes checked as well
• Pregnancy, which is why a pregnancy test is often the first step when periods have stopped

For the cleanest reading, collect in the morning at least an hour after waking, rested rather than rushed, and not straight after a big meal or a workout. If you find blood tests stressful, tell the collector; sitting quietly for a few minutes first helps.

What a mildly high result usually means, and when a doctor repeats the test

A prolactin a little above the range is one of the most common "abnormal" results in women's hormone testing, and one of the least likely to mean anything serious. Because the hormone is so responsive to stress and timing, the standard next step is to repeat it under calm conditions, sometimes with a short rest before the sample is taken. Many mildly raised results are within range on the second go.

If the repeat is still raised, your doctor works through the list above: medicines, thyroid, pregnancy. Laboratories also check a raised sample for macroprolactin, a form that registers on the test but is biologically inactive and does not affect your cycle.

Persistently and clearly raised prolactin with no everyday explanation is where a doctor looks further. A common explanation is a small, benign growth of prolactin-producing cells in the pituitary gland, called a prolactinoma; these are not cancer, are often tiny, and are well understood. Other growths near the pituitary, and conditions such as reduced kidney function, can raise prolactin too, which is why imaging is used to tell them apart. Very high levels, once the laboratory has excluded macroprolactin, are what point most strongly to a prolactinoma.

Prolactin in the KnowLuna hormone and fertility panels

Prolactin sits in every KnowLuna panel that looks at your cycle, so that a common, correctable reason for irregular periods or trouble conceiving is not missed.

• Thinking about fertility: the AMH and fertility panel checks prolactin alongside AMH, FSH, LH, oestradiol and TSH. Test-only $179, or $259 with a KnowLuna doctor to request and explain it.
• Irregular, heavy or changing cycles: the female hormone panel pairs prolactin with FSH, LH, oestradiol, progesterone, testosterone, SHBG, DHEA-S and cortisol. Test-only $139.

Collect at an accredited (NATA and RCPA) centre, from a network across Australia, or with a home kit or a nurse visit, on day 2 to 5 of your cycle if you have one and in the morning where you can. If prolactin is raised, a KnowLuna doctor follows up and talks through whether a repeat is the sensible next step. Browse the cycles and hormones stage for related tests, or read more on FSH and LH levels.

Common questions

Sources and further reading

• Pathology Tests Explained (RCPA): prolactin
• Healthdirect: pituitary gland
• Better Health Channel: pituitary tumour
• Jean Hailes for Women's Health: irregular periods

General information only, not medical advice. If your symptoms are severe or sudden, see a doctor promptly. This page does not replace a consultation with a doctor who knows your history; read our medical disclaimer.