A 32-year-old man presented with a 6-month history of severe headaches, hemiparesis on the left side, and impaired hearing in the left ear. His medical history was otherwise unremarkable.Magnetic resonance imaging(MRI) with gadolinium revealed a large mass (5.6 cm by 6.9 cm) invading the base of his skull (Panels A and B, arrows). To evaluate the patient for the presence of what was most likely a pituitary macroadenoma, the serum level of prolactin was measured and reported as 7.3 μg per liter (normal range, 4.1 to 18.4).
In cases of large prolactinomas, the initial prolactin level may be erroneously read as normal. After serial dilution of the patient's blood sample to 1:1000, the measured prolactin level was 122,260 μg per liter. Laboratory testing also revealed central hypogonadism. Immunohistochemical staining of a transnasal-biopsy specimen of the mass showed a lactotropic adenoma with tumor cells that were positive for prolactin. Four days after the initiation of treatment with cabergoline, the prolactin level declined to 10,823 μg per liter.
By the time of the 3-week follow-up visit, the prolactin level had further declined, to 772 μg per liter, and the patient's neurologic symptoms had resolved. After 40 months, the prolactin level was maintained at 25 μg per liter and the tumor had regressed substantially (Panels C and D, arrows). The central hypogonadism persisted.