Prolactin (PRL), also known as luteotropic hormone or luteotropin, is an hormone formed by a 199aa mature chain consisting of three disulfide bonds. Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation, and nursing. Prolactin is secreted in a pulsatile manner between these events. Prolactin also plays an essential role in metabolism, regulation of the immune system, and pancreatic development.

In normal individuals, prolactin concentrations increase in response to physiologic stimuli such as sleep, stress, exercise, sexual intercourse, and hypoglycemia, and are also elevated during pregnancy, lactation, postpartum, and in the newborn infant. Though, prolactin levels are found elevated  ven in non-pregnant and non-breastfeeding woman, or in male. The most common cause is a prolactinoma, a usually benign (not cancerous) prolactin-producing tumor of the pituitary gland.

The prolactin test may be used as part of a work-up for irregular menstrual periods, fertility problems, some types of thyroid or adrenal gland dysfunction, anorexia, and polycystic ovarian syndrome. Prolactin tests were also used to diagnose, or monitor treatment of prolactinoma. Symptoms of a prolactinoma include headaches, vision problems (if tumor growth is causing pressure on an the optic nerve), and galactorrhea (milk production outside pregnancy or breastfeeding, or in a male).