The prolactin blood test measures the level of a hormone produced by the pituitary gland, a small organ at the base of the brain. Healthcare providers frequently order this test to investigate hormonal imbalances affecting reproductive or overall health. The test provides a snapshot of prolactin production, helping diagnose symptoms like unexplained breast discharge, irregular menstrual cycles, or fertility issues.
Preparation for the Prolactin Test
Fasting is typically not required for a prolactin blood test, as food intake does not significantly affect the hormone’s concentration. However, if your doctor has ordered other tests concurrently, such as a glucose or lipid panel, they may still ask you to fast for eight to twelve hours. It is always important to confirm the specific requirements with your ordering physician or the laboratory performing the draw.
The timing of the blood draw is far more important than fasting because prolactin levels naturally fluctuate throughout the day, following a circadian rhythm. The hormone’s concentration is highest during sleep and shortly after waking. Therefore, the sample is ideally collected three to four hours after you have awakened. Scheduling the test for mid-morning, generally between 8 a.m. and 10 a.m., helps ensure the results reflect a standardized baseline level.
Minimizing physical and emotional stress immediately before the test is also a necessary preparation step. High levels of anxiety or even the anticipation of the blood draw can artificially elevate prolactin, a phenomenon known as the “stress response.” For this reason, many clinics recommend resting quietly for 20 to 30 minutes in the waiting area before the phlebotomist takes the sample.
You should also avoid strenuous physical activity for at least 24 hours before the test, as intense exercise can cause a temporary spike in prolactin levels. Additionally, avoid any form of breast or nipple stimulation, including sexual activity, for a full day prior to the blood collection. These activities can directly trigger the release of prolactin and lead to a misleadingly high result, potentially necessitating a repeat test.
The Role of Prolactin in the Body
Prolactin is a peptide hormone synthesized and secreted by specialized cells, called lactotrophs, in the anterior pituitary gland. The primary function of prolactin is directly related to reproduction and is most evident in women. Specifically, it stimulates the development of the mammary glands and initiates and maintains milk production after childbirth.
The hormone’s release is primarily controlled by the hypothalamus, which secretes dopamine to inhibit prolactin production. When dopamine’s inhibitory action is reduced, prolactin levels rise. Prolactin contributes to the regulation of the immune system and influences reproductive function in both sexes.
In women, prolactin helps regulate ovarian function and can inhibit the release of hormones necessary for ovulation, which is the body’s natural mechanism for preventing a new pregnancy during lactation. For men, prolactin is thought to influence testosterone production and plays a part in various metabolic functions.
Common Reasons for Elevated Prolactin
The most common reasons for a temporary elevation in prolactin are physiological and do not indicate a disease. Pregnancy and breastfeeding are the most significant natural causes, with prolactin levels rising dramatically to facilitate milk production. Levels can also increase temporarily due to emotional or physical stress, which triggers a generalized hormonal response.
Strenuous exercise, particularly activities performed immediately before the blood draw, has been shown to cause acute increases in prolactin. Similarly, sexual activity and direct stimulation of the chest wall or nipples can lead to a reflex release of the hormone. These temporary elevations are why the preparation guidelines are so specific regarding rest and avoiding physical exertion.
A number of prescription medications can also cause elevated prolactin levels by interfering with the brain’s dopamine signaling pathways. Common drug classes known to have this effect include certain antipsychotics, antidepressants, and some medications used to treat high blood pressure or gastrointestinal issues. Chronic conditions like hypothyroidism, where the thyroid gland is underactive, can also lead to an increase in prolactin.
Interpreting Prolactin Test Results
Normal prolactin levels vary by sex and are typically measured in nanograms per milliliter (ng/mL), though specific ranges can differ slightly between laboratories. For non-pregnant women, a normal result is generally less than 25 ng/mL, while for men, the level is usually below 20 ng/mL. Levels that consistently exceed the upper limit are known as hyperprolactinemia.
Hyperprolactinemia can cause a range of symptoms, especially in women, including galactorrhea (the production of breast milk unrelated to pregnancy), irregular or absent menstrual periods, and fertility challenges. In men, sustained high levels may be associated with decreased libido and erectile dysfunction.
When a high result is returned, a doctor will first evaluate whether the elevation was caused by one of the temporary factors, such as anxiety during the blood draw or a medication change. If the level remains high after a repeat test with careful preparation, the investigation may turn to pathological causes. The most common pathological cause is a prolactinoma, which is a benign tumor of the pituitary gland that secretes excess prolactin.
Other conditions that can cause sustained hyperprolactinemia include chronic kidney disease and diseases affecting the hypothalamus. If a prolactinoma is suspected, follow-up testing often includes magnetic resonance imaging (MRI) of the pituitary gland to visualize the structure. The results guide the treatment plan, which may involve medication to lower prolactin levels and shrink the tumor.