Giant Prolactinomas: Case Series and Review of the Literature

Giant prolactinomas are rare and constitute only 2-3% of prolactinomas. They are defined as prolactinomas greater than 4 cm in size with a prolactin level of > 1,000 µg/L. Unlike prolactinomas, giant prolactinomas have a male preponderance and present a decade earlier in men as compared to women. Giant prolactinomas may present with galactorrhea, irregular periods or decreased libido. Due to their large size, they can involve surrounding brain structures and may present with hydrocephalus, dizziness, seizures, deafness, and cognitive dysfunction. Laboratory assessment may reveal a falsely low prolactin level secondary to “hook effect” which is due to saturation of the capture and detection antibodies used in the assay. Dilution of the sample would show a paradoxical increase in prolactin levels confirming the hook effect. Magnetic resonance imaging (MRI) is warranted to assess the extent of the tumor. Dopamine agonists are the treatment of choice in giant prolactinomas and lead to rapid resolution of symptoms, normalization of prolactin levels, and reduction in the size of the tumors. Refractory giant prolactinomas may be treated with surgery, temozolomide or radiotherapy. Giant prolactinomas can pose unique diagnostic and management challenges because of atypical presentations and confounding laboratory assessments. We present four cases of giant prolactinomas each presenting in a unique manner and discuss the diagnostic and management dilemmas associated with them. J Endocrinol Metab. 2020;10(6):182-189 doi: https://doi.org/10.14740/jem652