Prolactin is a hormone that induces lactation or milk production. The size of the prolactinoma correlates with the excess prolactin level; the larger the tumor, the higher the level of prolactin that is secreted. Prolactinoma is the most common type of pituitary adenoma and is also referred to as a prolactin-secreting adenoma of the pituitary. These tumors account for at least 30% of all pituitary adenomas. At least half of all prolactinomas are microadenomas. Microprolactinomas are more common in women. Larger tumors, called macroprolactinomas, are more common in men. Many of the smaller tumors do not progress to larger tumors.
Prolactinomas occur most commonly in those under 40 years old. They are rare in children. These tumors are about fives times as common in women as men. In women, prolactinomas typically cause amenorrhea (lack of menstruation) and galactorrhea (milk production from the breasts, not associated with pregnancy) and occur in young patients. By contrast, prolactinomas in men occur in older patients and can grow to a large size before any symptoms occur.
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