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A prolactinoma is a tumor on the pituitary gland that makes too much of the hormone prolactin. This type of tumor is benign, which means it's not cancer.
The pituitary (say "puh-TOO-uh-tair-ee") gland—at the base of the brain—makes prolactin. After a woman is pregnant, this hormone causes the breasts to make milk. But a prolactinoma also makes prolactin. This means that your body can have too much of the hormone.
It's not normal for women who aren't pregnant or nursing to have a high level of prolactin. For both men and women, too much of this hormone can make the breasts produce milk. It also can cause low sex drive and infertility.