What is prolactin?

What is Prolactin

Prolactin is a protein hormone primarily secreted by lactotroph cells in the anterior pituitary gland that plays a central role in lactation and reproduction, while also influencing multiple other physiological systems including immune function, metabolism, and osmoregulation. 1, 2

Structure and Production

• Prolactin is a 23 kDa single-chain protein composed of 199 amino acids that is synthesized and released mainly by lactotrophs in the anterior pituitary gland 2
• The hormone exists in multiple posttranslational forms, including size variants and chemical modifications such as phosphorylation and glycosylation 3
• While the pituitary is the primary source, prolactin can also be produced by extrapituitary tissues including the central nervous system, immune system, uterus, and mammary glands 1, 4

Regulation of Secretion

• Prolactin secretion is predominantly under inhibitory control by hypothalamic dopamine, which acts as the major physiologic prolactin-inhibiting factor 1, 2
• The hormone regulates itself through a negative feedback loop, with prolactin providing the afferent signal for short-loop feedback control 2
• Prolactin-releasing factors include thyrotropin-releasing hormone (TRH), vasoactive intestinal peptide (VIP), and oxytocin 5
• Physiologic stimuli that increase prolactin secretion include sleep, stress, sexual intercourse, and suckling stimulus 5
• Estrogen increases serum prolactin levels, which explains why levels are higher in women, particularly during the late follicular phase and pregnancy 5

Primary Biological Functions

• The essential biological function of prolactin is stimulation of lactogenesis (milk synthesis) and galactopoiesis (maintenance of milk secretion) during pregnancy and lactation 1, 2
• During pregnancy, serum prolactin levels rise rapidly with increases in both the size and number of lactotrophs 2
• During lactation, suckling induces rapid prolactin secretion via a neuroendocrine reflex pathway 2

Additional Physiological Roles

• Prolactin influences reproductive function beyond lactation, including regulation of ovarian function and fertility 1, 5
• The hormone plays roles in angiogenesis, with a 16 kDa prolactin fragment demonstrating antiangiogenic and tumor growth inhibitory effects 4
• Prolactin affects immunoregulation and immune system function 1, 4
• The hormone contributes to osmoregulation and metabolic processes 1, 4
• Prolactin influences central nervous system function and can control various behaviors 3

Normal Physiologic Variations

• Prolactin levels vary with age and sex, being highest in the first 2 years of life, lowest in mid-childhood, and rising again during adolescence 6, 7
• Adolescent girls have higher prolactin levels than boys 6, 7
• Normal reference ranges for non-pregnant adults are typically <20 μg/L (or <425 mU/L), though women generally have higher baseline levels than men 7

Clinical Significance of Abnormal Levels

• Hyperprolactinemia causes reproductive dysfunction in both sexes, resulting in hypogonadism, infertility, and galactorrhea 1
• In women, elevated prolactin manifests as menstrual disturbances, amenorrhea, oligomenorrhea, and galactorrhea 7, 2
• In men, hyperprolactinemia causes decreased libido, erectile dysfunction, and gynecomastia 7
• Prolactinomas (lactotroph adenomas) represent approximately 40% of all pituitary tumors and are the most common pathological cause of chronic hyperprolactinemia 7, 1