Prolactin blood test: what it measures, normal levels, and what high prolactin levels mean

Prolactin is made by the front part of the pituitary gland, the pea-sized organ at the base of the brain. Its best-known job is to prepare the breasts for and sustain milk production after childbirth, but it circulates in everyone, and its control is unusual: the brain holds it down continuously with dopamine, so anything that lowers dopamine signaling — most notably many medications — lets prolactin rise. Prolactin in turn dials down the hormones that run the ovaries and testes (GnRH, then LH and FSH), which is why a high level shows up as a reproductive problem long before it shows up as milk.

The test is a single blood measurement of the hormone's concentration, reported in ng/mL (some laboratories use mIU/L; roughly 21 mIU/L equals 1 ng/mL). It is ordered for irregular or absent periods, infertility, unexplained breast-milk production (galactorrhea), low libido or erectile dysfunction in men, low testosterone, and when a pituitary tumor is suspected because of headaches or visual field changes. It is also checked when a pituitary problem is found incidentally on a scan.

Representative adult reference ranges are about 4–15 ng/mL in men and about 5–23 ng/mL in non-pregnant women; laboratories differ in method and upper limits (some report up to 25–29 ng/mL for women). Pregnancy raises prolactin steadily to several times the non-pregnant range by the third trimester, and breastfeeding keeps it high. Levels are highest during sleep and in the early morning and lowest in the late morning and afternoon, which is one reason a mildly high result is repeated before anything is concluded.

The degree of elevation is informative. Mild elevations (up to about twice the upper limit) are usually medication, stress, a meal, exercise, nipple stimulation, hypothyroidism or kidney disease. Levels above about 100 ng/mL usually mean a prolactin-secreting pituitary adenoma, and above about 250 ng/mL almost always do; the level roughly tracks the size of the adenoma. Two laboratory wrinkles can mislead: macroprolactin — prolactin bound to an antibody, biologically inactive but measured by the assay — can produce a high result in someone with no symptoms, and very large tumors can occasionally produce a falsely low reading on some assays (the 'hook effect') until the sample is diluted.

Ordered from most to least common in people who get tested:

In women the earliest sign is usually a change in the menstrual cycle — irregular, infrequent or absent periods — followed by difficulty conceiving, and sometimes galactorrhea (milk from the breasts when not breastfeeding), vaginal dryness and low libido. In men the picture is low testosterone: reduced libido, erectile dysfunction, infertility, loss of body hair, breast enlargement and, rarely, galactorrhea; because these develop slowly, men are often diagnosed later and with larger tumors. Both sexes can develop bone loss from the prolonged suppression of estrogen or testosterone. A large pituitary adenoma can also cause headaches and loss of peripheral vision by pressing on the optic nerves, and can crowd out the other pituitary hormones.

A high prolactin without any of these symptoms — particularly a modest one — is more likely to be a medication effect, a stress or timing artifact, or macroprolactin than disease, and the first step is to repeat it properly rather than to image.

Low prolactin rarely matters. Values below the range are seen with dopamine-agonist medications (cabergoline, bromocriptine — the very drugs used to treat high prolactin), and occasionally reflect general pituitary underactivity (hypopituitarism) after pituitary surgery, radiation, a severe postpartum hemorrhage (Sheehan syndrome) or a large tumor, in which case the other pituitary hormones are the concern. In women who have just given birth, a low prolactin can be a cause of failure to lactate. Outside those settings a low result needs no action.

Repeat the test under controlled conditions: fasting, mid-to-late morning, rested, having avoided exercise, sexual activity and nipple stimulation beforehand, and ideally with the needle placed and 15–20 minutes allowed before the blood is drawn so the stress of the draw itself does not raise the level. Review every medication and supplement with a clinician, and check TSH, a pregnancy test where relevant, and kidney function. If the level is only mildly raised and you have no symptoms, ask the laboratory to test for macroprolactin before any imaging.

If prolactin remains clearly elevated after those steps — and in any case if it is above about 100 ng/mL or there are symptoms such as headache or vision change — the next step is an MRI of the pituitary and an endocrinology referral. Prolactinomas respond well to dopamine-agonist tablets in most people, with prolactin normalizing and tumors shrinking; surgery is reserved for the minority that do not respond. In women planning pregnancy and men with low testosterone, treating the prolactin usually restores the reproductive hormones without separate hormone therapy.

Prolactin is a routine hormone test at Quest and Labcorp, orderable through a clinician or directly through TestWell with physician review, with results in a few business days. Draw it fasting in the mid-to-late morning, at least an hour after waking, without exercise, sex or breast stimulation beforehand, and bring your medication list to whoever reads it. It is commonly ordered with TSH (because hypothyroidism raises prolactin), and with FSH, LH and estradiol or testosterone when the question is periods, fertility or low testosterone — the TestWell hormone panels group these so one draw covers the work-up.