Misdiagnosis due to the hook effect in prolactin assay.

OBJECTIVES To describe a patient who was misdiagnosed as having a nonfunctional pituitary tumor due to the hook effect on prolactin measurements. CLINICAL PRESENTATION AND INTERVENTION A 45-year-old female was admitted with visual disturbances, panhypopituitarism and central diabetes insipidus due to pituitary tumor recurrence. She had been operated 4 times earlier and received cranial irradiation for a suspected nonfunctional pituitary adenoma. Serum prolactin was moderately elevated (164.5 ng/ml), but increased markedly after 1:100 dilution to 14,640 ng/ml. Diagnosis of a giant macroprolactinoma was made and cabergoline was started. Prolactin level normalized and a mild shrinkage of the tumor was achieved after 12 months of therapy. CONCLUSION The hook effect must be kept in mind while evaluating a giant pituitary adenoma with moderately elevated prolactin levels. This way unnecessary surgical procedures or irradiation may be avoided.