Prolactin ng/mL to mIU/L Converter

Prolactin in two units: the short answer

To convert prolactin from ng/mL to mIU/L, multiply by 21.2. To convert mIU/L back to ng/mL, divide by 21.2 (or multiply by 0.0472). A prolactin of 20 ng/mL is therefore about 424 mIU/L, and 500 mIU/L is about 23.6 ng/mL. Because ng/mL equals µg/L, results in µg/L read the same as ng/mL. The converter above handles both directions and shows typical upper limits for women and men.

Prolactin is one of the few common hormones where the SI-style unit is not a molar unit but an international unit (mIU/L) tied to a reference standard. That makes the conversion factor slightly different from one assay to another, and it explains why some sources quote 21.2 and others 20 or 21. This guide explains why, how to convert a result, what normal looks like, and why a raised prolactin is so often caused by something other than disease of the pituitary gland.

What prolactin does

Prolactin is a hormone made by the anterior pituitary gland. Its best-known role is stimulating breast milk production after childbirth, but it also has effects on the reproductive system, the immune system and metabolism. Unlike most pituitary hormones, prolactin is held in check mainly by inhibition: the hypothalamus releases dopamine, which travels to the pituitary and suppresses prolactin production. Anything that reduces dopamine's effect – a medicine that blocks dopamine, or a tumour pressing on the pituitary stalk – allows prolactin to rise.

High prolactin suppresses the release of gonadotropin-releasing hormone (GnRH), reducing LH and FSH. This is why raised prolactin can cause:

• in women: irregular or absent periods, infertility, milky nipple discharge (galactorrhoea) and vaginal dryness
• in men: reduced sex drive, erectile dysfunction, reduced testosterone, breast enlargement and, rarely, galactorrhoea
• in both sexes, over time: reduced bone density due to low sex hormones

ng/mL and mIU/L explained

• ng/mL (nanograms per milliliter) – the usual unit in the United States; identical to µg/L.
• mIU/L (milli-international units per liter) – used in the UK, Europe, Australia and many other countries. Some reports write µIU/mL, which is numerically identical to mIU/L.

Why the factor is 21.2

An international unit is defined by a reference preparation rather than by a molecular count. The current WHO reference material for prolactin, the Third International Standard (84/500), assigns 1 ng of prolactin an activity of about 21.2 µIU, so 1 ng/mL corresponds to 21.2 mIU/L. Older standards and different assays produce factors from roughly 20 to 36. Most modern laboratories use 21.2, but if your laboratory states a different factor on its reports, use theirs.

How to convert prolactin step by step

1. Check the unit: ng/mL, µg/L, mIU/L or µIU/mL.
2. For ng/mL (or µg/L) to mIU/L, multiply by 21.2.
3. For mIU/L (or µIU/mL) to ng/mL, divide by 21.2.
4. Round mIU/L to whole numbers and ng/mL to one decimal place.

Worked examples

• 15 ng/mL × 21.2 = 318 mIU/L – a typical upper limit for men.
• 25 ng/mL × 21.2 = 530 mIU/L – a typical upper limit for non-pregnant women.
• 50 ng/mL × 21.2 = 1,060 mIU/L.
• 100 ng/mL × 21.2 = 2,120 mIU/L.
• 250 ng/mL × 21.2 = 5,300 mIU/L – a level that usually points to a prolactinoma.
• 400 mIU/L ÷ 21.2 = 18.9 ng/mL.
• 1,000 mIU/L ÷ 21.2 = 47.2 ng/mL.

Mental shortcut

Multiply ng/mL by 20 and add 6% (or simply multiply by 21). For 30 ng/mL: 30 × 21 = 630; the exact answer is 636 mIU/L.

Prolactin conversion chart

Normal prolactin levels

Typical upper reference limits are about 25 ng/mL (530 mIU/L) for non-pregnant women and 15–20 ng/mL (320–425 mIU/L) for men, but these vary considerably by laboratory and assay. Many UK laboratories use upper limits of around 500–600 mIU/L for women and 350–400 mIU/L for men. Always use your laboratory's range.

Pregnancy and breastfeeding

Prolactin rises steadily throughout pregnancy, often reaching 200 ng/mL (4,240 mIU/L) or more by term, as oestrogen stimulates the pituitary. After birth, levels fall within a few weeks in women who do not breastfeed. In breastfeeding women, prolactin surges with each feed and baseline levels fall gradually over months. Prolactin testing in pregnancy and breastfeeding is rarely useful.

Children and newborns

Newborns have high prolactin from maternal oestrogen, falling over the first months. Children have low levels, which rise towards adult levels in puberty.

Causes of a high prolactin (hyperprolactinaemia)

A raised prolactin is common and has many causes. The Endocrine Society guideline recommends a careful search for non-tumour causes before attributing it to the pituitary.

Physiological causes

• pregnancy and breastfeeding
• stress, including the stress of a difficult blood draw
• exercise, sleep, sexual activity and nipple stimulation
• eating, particularly a high-protein meal

Medicines

Medicines are one of the most common causes. They include:

• antipsychotics – especially risperidone, paliperidone, amisulpride and haloperidol; aripiprazole tends to lower prolactin
• anti-sickness medicines – metoclopramide and domperidone
• some antidepressants – modest effects with SSRIs and tricyclics
• opioids, verapamil and methyldopa
• oestrogen in high doses

Antipsychotic-induced levels are often 25–100 ng/mL (530–2,120 mIU/L), but risperidone and amisulpride can cause higher levels.

Medical conditions

• Prolactinoma – a benign prolactin-secreting pituitary tumour; the most common functioning pituitary tumour.
• Other pituitary or hypothalamic tumours pressing on the pituitary stalk ("stalk effect"), usually causing modest rises (often below 100 ng/mL, 2,120 mIU/L).
• Primary hypothyroidism – raised TRH stimulates prolactin; always check TSH.
• Chronic kidney disease – reduced clearance.
• Liver cirrhosis.
• Polycystic ovary syndrome – mild rises in some women.
• Chest wall injury, shingles or surgery – nerve stimulation can raise prolactin.

How high is high?

The level gives clues to the cause. As a rough guide:

• Mild elevations (up to about 50–100 ng/mL, 1,060–2,120 mIU/L) – often due to medicines, stress, stalk compression, hypothyroidism or a small prolactinoma.
• Above about 150–250 ng/mL (3,180–5,300 mIU/L) – a prolactinoma is likely.
• Above about 500 ng/mL (10,600 mIU/L) – usually a large prolactinoma (macroprolactinoma).

Macroprolactin: the commonest false alarm

In some people, much of the prolactin in their blood is bound to antibodies, forming large complexes called macroprolactin. Macroprolactin is biologically inactive and stays in the blood longer, so it raises measured prolactin without causing symptoms. It accounts for roughly 10–25% of raised prolactin results in some series. Laboratories screen for it by polyethylene glycol (PEG) precipitation, reporting the "monomeric" (active) prolactin after removing macroprolactin. Guidelines recommend checking for macroprolactin when prolactin is raised in someone without typical symptoms. Many UK laboratories do this automatically.

The "hook effect"

With very large prolactinomas, prolactin can be so high that it saturates the assay, producing a falsely moderate result. When imaging shows a large pituitary tumour but prolactin is only modestly raised, laboratories repeat the test after diluting the sample. This matters because a large prolactinoma is treated with medication, whereas a non-functioning tumour may need surgery.

Treatment and monitoring

Prolactinomas are usually treated with dopamine agonists, mainly cabergoline, which lower prolactin and shrink the tumour in most people. Prolactin is checked periodically to adjust the dose, often aiming for a level within the reference range. Medicine-induced hyperprolactinaemia may be managed by switching to an alternative medicine if possible, under specialist advice – antipsychotics should never be stopped without the prescriber's guidance.

A step-by-step approach to a mildly raised prolactin

Most raised prolactin results are mild, and clinicians typically work through a sequence before ordering a pituitary scan:

1. Repeat the test under good conditions – mid-morning, rested, not after exercise, a large meal or breast stimulation. A single mild rise often normalises on repeat.
2. Check for pregnancy in anyone who could be pregnant.
3. Review all medicines, including antipsychotics, anti-sickness medicines, opioids and over-the-counter products.
4. Check thyroid function (TSH) and kidney function (creatinine/eGFR).
5. Check for macroprolactin, especially if there are no symptoms.
6. Consider an MRI of the pituitary when the level remains raised without another explanation, or sooner if it is markedly high or there are headaches or visual symptoms.

Prolactin and fertility

In women, a raised prolactin is a recognised and treatable cause of absent or irregular ovulation. Prolactin is therefore checked in women with irregular cycles, absent periods or galactorrhoea, although NICE fertility guidance advises against routine prolactin testing in women with regular cycles and no other symptoms. Treating hyperprolactinaemia with a dopamine agonist restores ovulation in most women. If a woman with a prolactinoma becomes pregnant, the medicine is often stopped, and she is monitored for symptoms of tumour growth such as headache or vision change rather than with prolactin levels, which rise naturally in pregnancy.

Prolactin in men

Men with prolactinomas often present later than women, because symptoms such as reduced libido and erectile dysfunction are less specific than menstrual changes and may be attributed to other causes. As a result, men are more likely to have large tumours (macroprolactinomas) at diagnosis, sometimes with headaches or vision problems. A man with low testosterone and low or normal LH should have prolactin measured. Treating the prolactinoma often restores testosterone without the need for testosterone therapy.

Prolactin and antipsychotic treatment

Raised prolactin is a common side effect of antipsychotic medicines. In the long term, it can cause sexual dysfunction, menstrual disturbance, breast symptoms and reduced bone density. Guidelines for people with psychosis often recommend measuring prolactin before starting treatment and when symptoms suggest hyperprolactinaemia. If prolactin is raised and causing problems, options include reducing the dose, switching to a prolactin-sparing medicine, or adding a low dose of aripiprazole – always under the supervision of the prescribing team. A baseline result before treatment is helpful, because it shows whether a later rise is due to the medicine.

Comparing results across laboratories

Because prolactin assays differ and use different factors, it is best to compare results from the same laboratory when tracking levels over time. If you must compare across countries, convert using 21.2 unless the report states a different factor, and compare each result with its own laboratory's range. For example, 22 ng/mL at a US laboratory with an upper limit of 23.3 ng/mL and 480 mIU/L at a UK laboratory with an upper limit of 496 mIU/L both sit just below the upper limit – consistent results despite the different units.

What affects prolactin results

• Stress and venepuncture – a difficult blood draw can raise prolactin. If a mild rise is found, some clinicians repeat the test after resting for 20–30 minutes with a cannula in place.
• Time of day – prolactin is higher during sleep and in the early morning; mid-morning samples are often preferred.
• Recent meals, exercise, sexual activity and breast examination.
• Medicines – see above.
• Macroprolactin – raises results without clinical significance.
• Assay differences – results can vary by 20% or more between platforms.
• Biotin may interfere with some immunoassays.

Low prolactin

A low prolactin is uncommon and usually not a problem in itself. It can occur with dopamine agonist treatment, with damage to the pituitary gland (for example after surgery, radiotherapy, or pituitary bleeding such as Sheehan's syndrome after severe blood loss at childbirth), and with some medicines. The most noticeable effect of low prolactin is difficulty producing breast milk after childbirth. A low prolactin together with low levels of other pituitary hormones suggests reduced pituitary function (hypopituitarism) and needs specialist assessment.

Typical prolactin levels at a glance

These figures are approximate and vary with assay; they are intended only to show orders of magnitude.

When to see a doctor

See a doctor if your prolactin is above the laboratory range, especially with irregular or absent periods, milky nipple discharge, reduced sex drive, erectile problems or difficulty conceiving. Seek prompt assessment for persistent headaches or changes in vision – particularly loss of peripheral vision – which may indicate a pituitary tumour pressing on the optic nerves. Sudden severe headache with vision loss or double vision is an emergency.