A prolactinoma is a very small, benign tumor in the pituitary gland that releases too much prolactin. It is the most common type of pituitary tumor. Symptoms of prolactinoma are caused by too much prolactin in the blood (hyperprolactinemia) or by pressure of the tumor on surrounding tissues. High prolactin can cause excessive production of breast milk or it can interfere with fertility in women or with sex drive and fertility in men. Damage to eye nerves is a complication of pregnancy for women with prolactinomas.
Autopsy studies indicate that 25 percent of the population of the USA have small pituitary tumors. Forty percent of these pituitary tumors produce prolactin, but most are not considered clinically significant.