title: Giant pituitary prolactinoma with falsely low serum prolactin: the pitfall of the "high-dose hook effect": case report.
abstract: OBJECTIVE
  We present a case of a giant prolactinoma with a falsely low serum prolactin concentration determined by a two-site chemiluminometric assay. Awareness of the possibility of the "high-dose hook effect" in such cases will prevent preoperative misdiagnosis.
  
  
  METHODS
  A patient with a giant invasive pituitary tumor was found preoperatively to have serum prolactin of 31 ng/ml. Pathological examination of the excised tissue, however, demonstrated strong staining for prolactin.
  
  
  METHODS
  Serial dilutions of the serum disclosed hyperprolactinemia of 280,000 ng/ml, establishing the presence of the "high-dose hook effect." Treatment with pergolide (Permax; Athena Neurosciences, S. San Francisco, CA) decreased tumor size and lowered serum prolactin by more than 99%.
  
  
  CONCLUSIONS
  A "high-dose hook effect" needs to be suspected in every patient with a giant pituitary adenoma (>3 cm) and normal serum prolactin. Subsequently assaying the diluted serum will reliably disclose this phenomenon and allow correct diagnosis and therapy.
authors.affiliation: Pituitary and Neuroendocrine Center, Department of Internal Medicine, University of Michigan Medical Center, Ann Arbor 48109-0354, USA.
authors.forename: A L
authors.initials: AL
authors.lastname: Barkan
authors.forename: W F
authors.initials: WF
authors.lastname: Chandler
mesh_terms: D000328:Adult; D018491:Dopamine Agonists / Q000627:therapeutic use; D006801:Humans; D008163:Luminescent Measurements; D008279:Magnetic Resonance Imaging; D008297:Male; D010479:Pergolide / Q000627:therapeutic use; D010911:Pituitary Neoplasms / Q000097:blood* / Q000188:drug therapy / Q000601:surgery*; D011388:Prolactin / Q000097:blood*; D015175:Prolactinoma / Q000097:blood* / Q000188:drug therapy / Q000601:surgery*
chemical_list: D018491:Dopamine Agonists; D010479:Pergolide; D011388:Prolactin