- Prolactin is a peptide hormone primarily produced by the lactotroph cells of the anterior pituitary gland, with smaller amounts synthesized in other tissues including the uterus, immune cells, and brain. Its most well-known role is in the regulation of lactation—the production and secretion of milk by the mammary glands—but prolactin also has a broad range of biological functions, impacting metabolism, reproduction, immune response, and behavior.
- The secretion of prolactin is unique among pituitary hormones in that it is predominantly under inhibitory control. The hypothalamus constantly suppresses prolactin release via dopamine, which acts as prolactin-inhibiting hormone (PIH). When dopamine inhibition is reduced, such as during pregnancy and postpartum periods, prolactin secretion rises significantly. In contrast to other anterior pituitary hormones like luteinizing hormone (LH) and follicle-stimulating hormone (FSH), prolactin is not regulated by a stimulatory hypothalamic hormone under normal physiological conditions.
- In females, prolactin plays a critical role in mammary gland development and milk synthesis. During pregnancy, levels of prolactin steadily increase, stimulating the growth and differentiation of mammary tissue. After childbirth, when progesterone and estrogen levels fall, prolactin stimulates the production of milk proteins such as casein and lactalbumin. The act of suckling by an infant stimulates nerve signals to the hypothalamus, reducing dopamine release and allowing further prolactin secretion in a positive feedback loop that helps sustain milk production.
- In males, prolactin is present at lower levels and its precise physiological role is less well-defined. It may modulate testosterone production, influence sexual behavior, and impact immune function. Excess prolactin in males, however, can lead to hypogonadism, reduced libido, erectile dysfunction, and infertility, largely due to the hormone’s inhibitory effects on the secretion of gonadotropin-releasing hormone (GnRH).
- Abnormal levels of prolactin are associated with various clinical conditions. Hyperprolactinemia (elevated prolactin levels) is the most common endocrine disorder of the hypothalamic-pituitary axis. It may result from prolactinomas (benign pituitary tumors), hypothyroidism, medications that block dopamine (such as antipsychotics), or chest wall injuries. Symptoms in women may include amenorrhea, galactorrhea (inappropriate milk production), and infertility, while men may experience gynecomastia, reduced testosterone, and sexual dysfunction. Conversely, prolactin deficiency, though rare, can impair lactation in postpartum women.
- Prolactin also has a multifaceted role in the immune system, where it acts as a cytokine-like molecule influencing lymphocyte proliferation and cytokine production. Some evidence suggests it may be involved in autoimmune conditions such as systemic lupus erythematosus (SLE) and rheumatoid arthritis, where elevated prolactin levels can enhance inflammatory responses.
- In therapeutic settings, dopamine agonists such as bromocriptine and cabergoline are used to treat hyperprolactinemia, effectively reducing prolactin levels and shrinking prolactin-secreting tumors. These agents mimic dopamine’s inhibitory effect on prolactin release from the pituitary.
