This patient presents with classic symptoms of a prolactinoma, i.e., amenorrhea and galactorrhea. Although the differential diagnosis for hyperprolactinemia is extensive, the most likely diagnosis in this patient is a prolactin-producing pituitary adenoma. It is critical to rule out pregnancy, as this is the most common physiologic reason for an elevated prolactin level. However, this patient is unlikely to be pregnant because she has had multiple negative pregnancy tests. Additionally, since she is otherwise healthy, it is unlikely that chronic renal failure is a cause of her elevated prolactin concentration. Two key clinical features that suggest a prolactinoma include the presence of headaches and a very high prolactin level. In general, prolactin levels greater than 100 to 200 ng/mL in a nonpregnant woman usually suggest a tumor instead of another cause.

Primary hypothyroidism is also important to consider, especially in women (who have a higher incidence of hypothyroidism than men). Primary hypothyroidism causes an elevation in prolactin concentration because thyrotropin-releasing hormone stimulates both thyroid-stimulating hormone and prolactin production from the pituitary gland. However, the most likely diagnosis for hyperprolactinemia in this patient is a pituitary tumor because she has had negative pregnancy tests, does not take any medications, has no symptoms to suggest chronic renal failure or hypothyroidism, and has a prolactin level >200 ng/mL.