Prolactin

Women: 102 to 496 mU/L · Men: 86 to 324 mU/L

Women run a little higher than men (roughly 100 to 500 versus 85 to 325 mU/L), the range is far higher in pregnancy and breastfeeding, and some labs use a single cut-off of around 500 or 600 for everyone.

Prolactin is a pituitary hormone whose main job is to stimulate milk production after childbirth. Outside pregnancy and breastfeeding it circulates at low levels in both sexes. When it runs high it can interrupt periods, lower sex drive, and cause milk production, so it is a common test in fertility and hormone clinics.

Doctors request it for absent or irregular periods, unexpected milk production, fertility problems, low sex drive or erectile problems, low testosterone in men, and headaches or visual symptoms where a pituitary growth is being looked into.

• Stress, pain, a difficult blood draw, recent exercise or a recent meal
• Many medicines, especially antipsychotics, anti-sickness tablets, some antidepressants and some blood pressure tablets
• Pregnancy and breastfeeding
• An underactive thyroid
• A benign pituitary growth called a prolactinoma, or other pituitary conditions
• Long-term kidney disease
• Macroprolactin, a harmless large form that the assay picks up

• Dopamine-type medicines used for Parkinson's disease and for high prolactin
• Rare pituitary failure
• It is naturally at its lowest in the late morning and early afternoon

• Stress, exercise, a recent meal, breast examination or nipple stimulation before the test can all raise it
• Macroprolactin, a harmless bound form, accounts for a fair share of raised results and most labs screen for it automatically
• It is highest overnight and in the early morning
• Common medicines, including anti-sickness tablets, raise it

Best taken mid-morning, at least an hour after waking, rested and not straight after exercise, a meal or breast examination. A single raised value is usually repeated. Labs often check for macroprolactin when the level is raised and print a 'monomeric' result.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.