A 30-year-old woman came to a doctor with complaints of milk discharge from her breasts and no menstruation for the last 5 months. She had one physiological childbirth four years ago. The development of her breasts is normal. Bimanual examination detects a reduced in size uterus and normal size of the ovaries. MRI detects no cerebral pathology. Thyrotropin levels are within the normal range, prolactin levels in blood serum are elevated. What is the most likely diagnosis in this case?