High-dose Hook Effect in a Case of Giant Prolactinoma Confounded by Acute Kidney Injury

A 21-year-old man admitted to the hospital for acute kidney injury developed blindness 5 days after admission. Workup included magnetic resonance imaging, which was terminated prematurely due to a ball bearing/air gun pellet in his neck causing pain. His incomplete scan revealed a large sellar mass causing significant compression of the optic chiasm. Hormone testing showed low levels of IGF-1 and testosterone. A mildly elevated prolactin (PRL) level was attributed to stalk effect and decreased kidney function. For treatment, the patient underwent transsphenoidal tumor resection. Unexpectedly, pituitary immunohistochemistry revealed a prolactinoma. Further inquiry corroborated a history of headache, hypogonadal symptoms, and gynecomastia. He was started on a dopamine agonist with improvement in his vision and hormone levels; however, PRL was further increased. In retrospect, the mildly elevated PRL on initial testing was caused by the high-dose hook effect whereby excessively high PRL levels result in erroneously low measurements by immunoassays. Thus, in patients presenting with large sellar masses and normal/mildly elevated PRL, the sample should be diluted to exclude high-dose hook effect and ensure an accurate level. In addition, questioning of metallic fragments in plain language may help prevent magnetic resonance imaging injury.