The Hook Effect: A Case Study of a Giant Invasive Prolactinoma With Falsely Low Serum Prolactin

Prolactinomas are benign pituitary tumors also known as prolactin-secreting adenomas (PSA). These tumors cause excessive secretion of prolactin (hyperprolactinemia), a hormone responsible for lactation. Diagnosing hyperprolactinemia relies on measuring prolactin levels in the blood, and elevated serum levels of prolactin are typically indicative of prolactinoma. The hook effect occurs in immunological tests such as the prolactin level test. When the amount of prolactin present in the sample is too high and exceeds the binding capacity of the antibodies being used, the test result may indicate falsely low levels of prolactin, which is the hook effect. The present study describes the case of a male patient who presented with neck pain and difficulty swallowing. MRI revealed a giant (>40mm) extradural tumor affecting the clivus, anterior fossa, pterygopalatine, and bilateral infratemporal fossae as well as the petrous apex and bilateral cavernous sinuses. Endocrinological investigation yielded no specific abnormalities. An occipitocervical fixation (arthrodesis) was proposed with simultaneous extended endoscopic endonasal resection. Surgery succeeded in resecting a portion of the clival tumor and the anterior fossa. Measurement of prolactin levels several weeks post-surgery found them to be extremely high, confirming the hook effect.