Prolactinoma is a common cause of high prolactin levels and its subsequent symptoms, but other conditions may have similar effects. For example, other types of pituitary tumors (including those that are cancerous) and thyroid problems may affect hormone levels. Pregnancy and breast feeding are normal, healthy conditions that increase levels of prolactin and affect the balance of other hormones. Certain medications may also cause too much prolactin and lead to other symptoms. If you experience symptoms of prolactinoma, it's important to see a doctor to determine the cause.