Successful diagnosis and monitoring of giant prolactinomas: the role of sample dilutions.

Introduction: Giant prolactinoma, a prolactin-secreting pituitary adenoma greater than 4 cm in largest dimension, poses unique diagnostic and therapeutic challenges, setting it apart from smaller prolactin-secreting tumors.

Methods: A 48-year-old man reported decreased ability to maintain penile erection. His genital examination was normal. Massively elevated prolactin (17 164.5 µg/L, confirmed by dilution), low testosterone, and low luteinizing hormone were found. A giant prolactinoma encasing the internal carotid arteries and invading the surrounding bones was found. Carotid flow voids were not narrowed. Formal visual field testing revealed no deficits.

Results: The insulinlike growth factor 1 was normal, and the growth hormone suppressed appropriately 120 minutes after a 75-g oral glucose load. Over 8 months of cabergoline, the prolactin level normalized and the testosterone level increased. Striking regression of the tumor was observed after 1 year of therapy, although prolonged therapy was planned given that the mass was still encasing the carotid arteries. The patient's sexual symptoms improved. All prolactinomas should be treated with dopamine agonists to lower prolactin levels, shrink the tumor, preserve vision (if compromised), and restore gonadal function.

Discussion: This case of giant macroprolactinoma highlights the need for confirming markedly elevated prolactin measurements with dilution of the sample; assessing for concurrent growth hormone excess; and initiating dopamine agonist therapy as a first line, long- term therapy.