The hormone prolactin is secreted from the pituitary gland at the base of the brain. Its main function in humans is milk production. If prolactin is high, it may interfere with reproductive function. High prolactin may cause lack of periods or irregular periods or milky breast discharge. High prolactin may be due to a benign pituitary tumor, stress, sexual intercourse, or certain medications. When breast discharge or irregular periods occur, a blood test for prolactin should be performed. If prolactin is elevated, an x-ray of the pituitary gland (CT scan or MRI) is performed to rule out a tumor called pituitary adenoma (or prolactinoma). If a small tumor is found, it is often treated with medication alone and followed closely. Most of these will either shrink or remain unchanged with medications. If a large tumor is found, a referral to a specialist (neurosurgeon) is required.