A prolactinoma is a tumor of the prolactin-producing cells of the pituitary gland. The pituitary gland, or master gland, is located at the bottom of the brain and controls growth, metabolism, and reproduction. One of the many hormones secreted by the pituitary gland is prolactin. Prolactin causes a woman's breasts to secrete milk after pregnancy. Tumors in the prolactin-secreting cells of the pituitary cause too much prolactin in the bloodstream. Prolactinomas are rare and occur in 1 or 2 people out of 10,000. Most are treatable.