What is a Normal Prolactin Level?

For five years, my prolactin has ranged as high as 29. I had an MRI five years ago that found no abnormal tumor. My former doctor said my reading of 29, which was two months ago, fell within the "normal" range of 3-30. However, I just started seeing a new doctor who said that my latest reading of 26.4 is in fact high, and that it is highly unusual. He says I should now see an endocrinologist.


What is the normal range for prolactin for someone age 33 who has never been pregnant? I have had irregular periods since I was 19, and I have been exercising regularly for the last nine years.


Your question about normal prolactin levels necessitates more than one answer. Let's first look at prolactin. This protein is produced by the pituitary gland and also by the uterine lining. Its best-known function is to promote milk production in lactating (breastfeeding) women. Elevated prolactin levels (called hyperprolactinemia) may interfere with ovulation and menstrual cycle regularity. They may also cause galactorrhea (inappropriate milk secretion) and decreased libido (sex drive).

Many conditions may alter prolactin secretion. Breast stimulation, sexual intercourse, pelvic examinations and stress can all cause minor elevations of prolactin. Medications, particularly antipsychotic medications, may cause more significant elevations.
Most pituitary hormone production is controlled by signals that come in the form of other hormones. That means the pituitary acts only when it receives a stimulating hormone message from elsewhere in the body. For example, when thyroid activity is low, the hypothalamus, a higher brain center, releases the hormone TRH to stimulate the pituitary to release thyroid-stimulating hormone TSH. (Surprisingly, the hormone TRH, in addition to stimulating the pituitary release of TSH, also triggers the release of prolactin.)

Prolactin is a bit different from all other pituitary hormones. In most cases, production of pituitary hormones is normally turned "off," and releasing hormones are sent to stimulate the pituitary when more pituitary hormones are needed. In the case of prolactin, however, prolactin secretion is generally turned "on" unless the hypothalamus secretes the prolactin-inhibiting hormone dopamine to turn prolactin production "off." Dopamine travels from the hypothalamus to the pituitary in a small network of veins called a venous portal system. Anything that interferes with this fine network may prevent this inhibitory message from reaching the pituitary gland. The result is that the pituitary will produce too much prolactin. In some cases this overstimulation induces the overgrowth of prolactin-producing cells, and a small pituitary tumor (adenoma) is the result.
When the hypothalamus sees high prolactin levels, it manufactures more dopamine, trying to get prolactin levels back to normal.

Unfortunately, if the message doesn't get through, the pituitary continues to produce and release prolactin. Meanwhile, the high levels of dopamine in the hypothalamus can block the release of another hormone, GnRH, which is necessary for normal ovarian function.

Do you need to be worried by the prolactin blood levels you have reported? We need more information to answer that question. First, you need to know that any lab reading is only an approximation of the real hormone blood level. I can see the confused look on your face, so let me explain further. There is an actual level of hormone in your blood. We don't know what that value is, so we order a test. Unfortunately, there are many different types or brands of blood hormone tests. When a manufacturer designs the lab test, it sets up the test by using it on samples with known values of hormone. So tube one has a level of 10, tube two level 20, tube three level 50, etc. The test is run, and each tube gives a specific number. These results are plotted on a graph. Later, when an unknown sample is tested, the result is measured against the graph to determine the level of hormone in the test sample.

Theoretically, we should get reasonably similar results when your blood sample is sent to different labs. But it is not that simple. Remember we started with tubes with known hormone amounts -- 10, 20, 50? These tubes are called "hormone standards." Since all blood tests compare your sample against other known samples, we rely on these standards to be truly similar between different brands of the test. Unfortunately, they are not. So what is considered 10ng/ml of prolactin in one test's standard may not be similar to 10ng/ml in another test. Yes, it is confusing. The result is that one test brand may indicate one value, while a different test may return a value that is quite different. So if I send your blood to 10 different labs, I will most likely get 10 different results that may vary as much as 100 percent.

This means I cannot really tell whether a given value is normal. Next time you look at a laboratory report, you will also note that normal ranges are specified next to your test result. That means that any sample above or below certain amounts will be in the normal range for a male or female your age. So while a value of 30 may be normal in one lab, 26 may be high in another.

Regardless of all this uncertainty in testing, the values you cite are still quite low. We are rarely concerned about pituitary adenoma unless the values are above 100ng/ml. My suggestion would be to consider alternative explanation and treatment for your menstrual irregularity.

by Mark Perloe