Prolactin Levels Explained: High Prolactin Causes

Prolactin is a hormone made by the pituitary gland. Its best-known job is to stimulate milk production after birth, but it also affects periods, fertility, sex drive and testosterone. When prolactin is too high, it can switch off the hormones that drive ovulation and sperm production.

This guide explains typical prolactin ranges, the many everyday reasons a result can be high, including the stress of the blood test itself, what macroprolactin is, the symptoms and causes of high prolactin, and when to get urgent help.

Test it in Chiswick: our Prolactin blood test costs £49, with your result explained in person by a practitioner. No GP referral needed. Book this test or call 020 4515 0000. Prefer to test at home? A home finger-prick kit is also available, ask us when you enquire.

Quick answer

Prolactin is measured in mIU/L (also written mU/L). Two NHS laboratories quote about 109 to 557 mIU/L for women and 73 to 407 mIU/L for men. NHS laboratories routinely check results above 700 mIU/L for macroprolactin, a harmless form. Common causes of a raised level include pregnancy, breastfeeding, stress (including the blood test), some medicines such as antipsychotics, an underactive thyroid and a prolactinoma. A mildly raised result is usually repeated before any conclusions are drawn.

What prolactin does

The Society for Endocrinology explains that prolactin prepares the breasts during pregnancy and supports milk production afterwards. Its release is held in check by dopamine from the brain. Anything that blocks dopamine, or presses on the pituitary stalk, can let prolactin rise. High prolactin lowers the signals that control ovulation and testosterone, which is why it can cause missed periods, low sex drive and fertility problems.

What your prolactin result means

Reference ranges differ between laboratories. The ranges below come from NHS laboratories using an Abbott assay, and the bands above 700 mIU/L come from an NHS laboratory guide for GPs (Royal United Hospitals Bath). Read your result against the range on your own report.

• Typical range, women: about 109 to 557 mIU/L.
• Typical range, men: about 73 to 407 mIU/L.
• Above 700 mIU/L: NHS laboratories screen the sample for macroprolactin. Up to 2,000 mIU/L is described as mild hyperprolactinaemia, and a repeat test in calm conditions, avoiding stress and other temporary causes, is usually the next step.
• 2,000 to 5,000 mIU/L: described as significant hyperprolactinaemia, which needs prompt GP review and often an endocrinology opinion.
• Above 5,000 mIU/L: described as severe, which needs prompt specialist (endocrinology) assessment.

Causes of high prolactin

Everyday and physiological causes

• Pregnancy and breastfeeding: prolactin rises naturally, so routine testing is not recommended at these times.
• Stress, including venepuncture: the Society for Endocrinology notes that even having blood taken can raise prolactin.
• Sleep, exercise and nipple stimulation: all can raise prolactin temporarily.

Medicines

• Antipsychotics: a common cause, as they block dopamine.
• Anti-sickness medicines: such as metoclopramide and domperidone.
• Some antidepressants, opioids and high-dose oestrogen: can also raise prolactin.

Never stop a prescribed medicine because of a prolactin result. Speak to the prescriber first.

Medical conditions

• Underactive thyroid (hypothyroidism): can raise prolactin, so thyroid tests are usually checked too.
• Prolactinoma: a benign pituitary tumour that makes prolactin. Most respond well to tablets.
• Other pituitary tumours: can cause a modest rise by pressing on the pituitary stalk.
• Kidney disease, severe liver disease and PCOS: are also listed by NHS laboratories.

What is macroprolactin?

Macroprolactin is prolactin bound to an antibody, forming a large complex that shows up on the test but has minimal activity in the body. NHS laboratories automatically screen prolactin results above 700 mIU/L by treating the sample to remove it and re-measuring. The Society for Endocrinology notes that people whose raised result is due to macroprolactin need no further tests or treatment.

Symptoms of high prolactin

• Women: irregular or absent periods, difficulty conceiving, milky nipple discharge (galactorrhoea), and symptoms of low oestrogen.
• Men: low sex drive, erection problems, reduced fertility and, sometimes, breast enlargement.
• Both: large prolactinomas can press on nearby nerves and cause headaches or problems with vision. Long-term high prolactin can also thin the bones.

When to see your GP or get urgent help

Call 999 or go to A&E if you have a headache that comes on suddenly and is extremely painful, or a severe headache with loss of vision or other sudden vision problems. The NHS gives this advice for a sudden, extremely painful headache or one with loss of vision, and the Society for Endocrinology describes pituitary apoplexy, a sudden bleed into a pituitary tumour causing severe headache, vomiting and visual problems, as a medical emergency.

See your GP if you have:

• Headaches that keep coming back, or gradual changes in your vision such as losing side vision.
• Milky nipple discharge when you are not pregnant or breastfeeding.
• Missed three periods in a row, or new irregular periods.
• Low sex drive or erection problems, or a raised prolactin result on a repeat test.

Preparing for your prolactin test

• Stay calm and rested: stress, recent exercise and a difficult blood draw can raise prolactin. Avoid hard exercise beforehand and allow time to sit quietly before your sample.
• Time of day: an NHS laboratory guide says a sample taken without excessive stress is valid at any time of day.
• Fasting: not usually required, although one NHS guide notes a non-fasting sample can be slightly higher.
• Medicines and supplements: bring a list of your medicines, and mention biotin, which can affect some hormone tests.
• Pregnancy: tell us if you could be pregnant or are breastfeeding.

How often to retest

A mildly raised first result is usually repeated, often a few weeks later, in calm conditions. Higher or persistently raised results need GP or specialist review, and people being treated for a prolactinoma are monitored with regular blood tests as advised by their specialist.

Frequently asked questions

Is a prolactin of 800 high?

Yes, 800 mIU/L is above typical ranges for women and men. It is high enough for NHS laboratories to screen for macroprolactin, and a repeat test in calm conditions is usually advised. Many results at this level are explained by stress, medicines or macroprolactin.

Can stress raise prolactin?

Yes. Physical or emotional stress, including the blood test itself, can raise prolactin. This is one reason a single mildly raised result is repeated.

What is macroprolactin, and is it harmful?

It is a large, antibody-bound form of prolactin with minimal activity in the body. It can make your result look high, but on its own it is not harmful and needs no treatment.

Can high prolactin stop my periods?

Yes. High prolactin lowers the hormones that trigger ovulation, which can make periods irregular or stop them, and can make it harder to conceive.

Should I test prolactin if I am trying to conceive?

NICE advises that prolactin testing is offered for fertility concerns only when there is an ovulation problem, milky nipple discharge or a pituitary tumour. If your periods are irregular, a test can be helpful.

Prolactin testing in Chiswick

Our clinic is at 8 to 9 Bedford Corner, The Avenue, Chiswick, London W4 1LZ, a short walk from Turnham Green station (District line; Piccadilly line trains stop there early morning and late evening only), and close to buses 94, E3 and 272. Limited parking is available on request when you book. Your sample is taken at the clinic and analysed at the Randox accredited laboratory, and most results are ready within a few working days. You do not need a GP referral, a practitioner explains your result with you, and a GP feedback report is available on request.

Book the prolactin blood test on its own, or choose the Hormone Health Check (£69), which adds FSH, LH, oestradiol, progesterone, testosterone and SHBG. You can also compare every blood test and price, or read our guides to thyroid function test results and testosterone levels in men.

This article is general information, not medical advice. Discuss your results with your GP or practitioner.