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A prolactinoma is the most common type of pituitary tumor. It is a benign tumor of the pituitary cells that produce prolactin, resulting in high levels of prolactin in the blood (hyperprolactinemia).
Prolactin stimulates the breast to produce milk during pregnancy. After delivery of the baby, a mother's prolactin levels fall unless she breast feeds her infant. During nursing, prolactin levels rise to maintain milk production.