A 50-year-old man has progressive headache and bitemporal visual field loss. MRI shows a 3 cm sellar mass compressing the optic chiasm. Serum prolactin is 85 ng/mL (normal < 20). Which statement best explains this prolactin value?

• A It represents a prolactin-secreting macroadenoma, since any level above 20 ng/mL confirms lactotroph origin
• B It reflects stalk effect: compression of the pituitary stalk by a nonfunctioning adenoma removes dopaminergic inhibition of normal lactotrophs ✓
• C It indicates ectopic prolactin secretion by the mass, as pituitary tumors never produce prolactin indirectly
• D It is spurious macroprolactin and requires no further evaluation before surgery

Explanation

Dopamine from the hypothalamus tonically inhibits prolactin secretion via the portal system. A large nonfunctioning adenoma can compress the stalk, cutting off dopamine delivery and causing modest hyperprolactinemia, usually below about 200 ng/mL. True prolactin macroadenomas typically produce levels far higher, often above 200 ng/mL, proportional to tumor size. This distinction matters clinically because stalk effect lesions are treated surgically, while prolactinomas are treated medically with dopamine agonists such as cabergoline.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.