A 30-year-old woman complains of milk discharge from her breasts and no menstruation for the last 5 months. One physiologic childbirth was 4 years ago. There are no maldevelopments of mammary glands. Bi-manual examination revealed diminished uterus and normal sized ovaries. MRI-scan shows no brain pathologies. Thyroid-stimulating hormone is within normal limits. Serum prolactin is high. What is the most likely diagnosis?