Prolactin is a peptide hormone produced primarily by the pituitary gland, a small structure located at the base of the brain. Its primary function is involvement in reproductive processes, particularly stimulating the production of milk in females. Although often associated with pregnancy and breastfeeding, prolactin influences the health of both men and women throughout their lives. Understanding what constitutes a “normal” prolactin level is necessary because the reference range changes significantly based on sex, age, and reproductive status. This article explores the hormone’s biological actions and provides the reference levels used by healthcare providers to assess health across various life stages.
Primary Physiological Roles of Prolactin
The most recognized role of prolactin is its direct action on the mammary glands, which involves two main stages. Prolactin stimulates the growth and development of mammary tissue (mammogenesis), particularly during puberty and pregnancy. Its primary role is lactogenesis, the initiation and maintenance of milk production following childbirth. Prolactin also influences reproductive health by affecting sex hormones. In women, elevated prolactin suppresses the release of gonadotropin-releasing hormone (GnRH), which inhibits ovulation and the menstrual cycle. This mechanism helps prevent pregnancy while nursing. Prolactin also affects libido in both sexes and is involved in modulating the immune system and metabolism.
Normal Prolactin Reference Levels by Life Stage
The reference range for prolactin is highly dependent on a person’s physiological state, with levels measured in nanograms per milliliter (ng/mL) or micrograms per liter (\(\mu\)g/L). For non-pregnant, premenopausal women, the typical normal range is approximately 3 to 30 ng/mL. These levels fluctuate slightly throughout the menstrual cycle, often rising around ovulation. During pregnancy, prolactin levels increase dramatically to prepare for breastfeeding, often reaching 10–209 ng/mL by the third trimester. Postmenopausal women typically have lower levels, often falling within the range of 2 to 20 ng/mL, due to decreased estrogen.
For men, prolactin levels are generally lower than in women, with a common reference range being 2 to 18 ng/mL. These low levels are necessary for normal sexual function and testosterone production. In children and adolescents, prolactin levels are low before puberty but show sex-specific differences during sexual development. For example, adolescent males in later puberty (Tanner stages 4-5) may see levels between 2.8 and 11 ng/mL, while adolescent females may range from 3.2 to 20 ng/mL. Specific reference ranges can vary between different laboratories and testing methods.
Causes and Symptoms of Elevated Prolactin
Hyperprolactinemia occurs when prolactin levels are consistently above the normal range in non-pregnant, non-lactating individuals. The most common cause of this chronic elevation is a prolactinoma, a benign tumor of the pituitary gland that overproduces prolactin. These tumors secrete excessive hormone amounts, sometimes leading to levels in the hundreds or thousands of ng/mL. Other medical conditions can also cause hyperprolactinemia by interfering with the brain’s regulatory signals.
Primary hypothyroidism, an underactive thyroid, elevates prolactin because the increased thyrotropin-releasing hormone (TRH) also stimulates prolactin release. Chronic kidney disease is another cause, as it impairs the body’s ability to clear prolactin from the bloodstream. Symptoms of chronically elevated prolactin relate primarily to the suppression of sex hormones (estrogen and testosterone). In women, this suppression often leads to galactorrhea (production of breast milk outside of pregnancy) and menstrual disturbances. These disturbances include irregular periods (oligomenorrhea) or the complete absence of periods (amenorrhea), potentially resulting in infertility and reduced bone density. Men typically experience decreased libido, erectile dysfunction, and infertility due to suppressed testosterone, and may also develop galactorrhea.
Non-Pathological Factors Affecting Prolactin Measurement
An unexpectedly high prolactin result does not always indicate a chronic medical condition, as the hormone is sensitive to temporary physiological changes. Prolactin secretion follows a diurnal rhythm, with levels naturally rising during sleep. Therefore, blood samples should be drawn a few hours after waking to capture a basal level. The physical and emotional state surrounding the blood draw can significantly influence the result.
High levels of stress, including the anxiety associated with the venipuncture itself, can cause a temporary spike in prolactin secretion. For this reason, a healthcare provider may request a repeat test if the initial result is only mildly elevated and the patient was visibly distressed. Other factors that trigger a transient increase include strenuous exercise, nipple stimulation, or recent sexual activity. Certain common medications also block dopamine, the main inhibitor of prolactin, thereby causing elevated levels. These include some antipsychotic drugs, specific antidepressants, and certain anti-nausea medications. For the most accurate measurement, patients should fast, avoid intense physical activity before the test, and ensure the blood draw occurs after a period of rest, ideally 3 to 4 hours after waking.