Prolactin is a hormone produced in the pituitary gland and various other sites elsewhere in the body, including the brain, placenta, uterus, mammary gland, and lymphocytes of the immune system.
Prolactin has many roles that are important for reproduction. It is best known for the effects on the mammary gland, which include stimulating the growth and development of the mammary gland (mammogenesis), synthesis of milk (lactogenesis), and maintenance of milk secretion (galactopoiesis). Prolactin influences luteal function with abnormal prolactin levels affecting the normal luteal phase of the menstrual cycle, as well as the normal production of estradiol and progesterone.
Prolactin is produced in skin and hair follicles, playing important roles in the regulation of keratin expression, promotion of hair growth, and control of epithelial stem cell function. Prolactin is also important for normal bone growth, as elevated levels are associated with decreased bone mineral density and increased risk of vertebral fractures. In addition, numerous links have been reported between prolactin, the environment, and psychological stress, illustrating a role for prolactin in the stress response.
Prolactin also plays roles in the regulation of the immune system, osmotic balance, angionenesis, metabolic homeostasis (including body weight control), fat tissue formation, pancreatic function, and maternal behaviour.