Prolactin is a hormone secreted by the pituitary gland in both men and women. In women, it prepares breasts to produce milk (lactate). In men, prolactin's function is not well understood.
In a lactating woman, more prolactin is produced when her breasts are emptied frequently, resulting in more milk production. Less frequent emptying of the breasts reduces the amount of prolactin produced, which in turn reduces the milk supply. Above-normal prolactin levels in women can cause ovulation to stop.
Above-normal levels of prolactin can indicate a problem with the pituitary gland that may affect reproductive functions.