Does an elevated serum PRL level indicate that a tumor is a functioning prolactinoma?
No. The secretion of PRL is negatively regulated by hypothalamic dopamine (DA) via the pituitary stalk. Stalk compression from a large nonfunctioning tumor can impair DA delivery and, thus, increase PRL levels. The serum PRL rarely exceeds 100 to 150 ng/mL in such cases, whereas it is usually much higher with PRL–secreting tumors (e.g., PRL levels > 500 ng/mL with macroprolactinomas). In rare cases when the distinction between a prolactinoma and a NFA is difficult, a brief empiric trial of a DA agonist can be considered. The hallmarks of a NFA with hyperprolactinemia from stalk effect are that the PRL level often becomes undetectable (< 1 ng/mL) on even low-dose cabergoline, and the tumor does not shrink appreciably even after several months of DA treatment.