A prolactinoma is a benign (noncancerous) tumor of the pituitary gland that produces a hormone called prolactin. Located at the base of the brain, the pituitary is a pea-sized gland that controls the production of many hormones. Prolactin signals a woman’s breasts to produce milk during pregnancy and breastfeeding.
https://www.niddk.nih.gov/health-information/endocrine-diseases/prolactinoma

