Giant invasive pituitary prolactinoma with falsely low serum prolactin: the significance of ‘hook effect’

The authors report a case of a patient with giant, invasive skull base tumor extending to the parasellar area discovered incidentally during the work-up for decreased memory. The patient’s neurological exam was otherwise unremarkable. Endocrine evaluation performed at a local hospital showed a moderate hyperprolactinemia 103 ng/ml (normal up to 20 ng/ml). Given the large size of the tumor, the elevated prolactin (PRL) was interpreted to be secondary to stalk effect and patient underwent debulking surgery through a transcranial approach. Immunostaining of the excised tumor tissue was strongly positive for prolactin. His prolactin was found to be 13,144 ng/ml in our lab after surgery confirming the diagnosis of invasive giant prolactinoma. The patient developed a complete right third, fourth and sixth nerve palsy postoperatively. He was started on Cabergoline with normalization of his prolactin level and more than 50% decrease in residual tumor size over 9 months periods. There has been no clinically significant improvement in his right eye ophthalmoplegia since surgery. This case highlights the importance of ‘Hook Effect’ resulting in falsely low prolactin level, which may have significant therapeutic implication.