Ultra-rapid prolactin normalization with cabergoline in a premenopausal woman with epilepsy and giant prolactinoma

Giant prolactinomas are rare pituitary tumors characterized by extremely high prolactin levels and extensive extrasellar invasion. While dopamine agonists are usually effective, they may require months to normalize prolactin. We report a 47-year-old premenopausal woman presenting with new-onset temporal epilepsy and secondary amenorrhea. Assessments revealed marked hyperprolactinemia (mean serum prolactin concentration, 3902 ng/mL [SI: 169.8 nmol/L]) (reference range, 5.0-30.0 ng/mL [SI: 0.22-1.30 nmol/L]), and an invasive 47 × 43 × 50 mm adenoma extending into the right temporal lobe. After initiating cabergoline (0.25 mg/day), serum prolactin dropped to 802 ng/mL (SI: 34.9 nmol/L) within 18 hours and normalized by day 4 (12.7 ng/mL [SI: 0.55 nmol/L]). Polyethylene glycol recovery excluded macroprolactinemia. Despite ultra-rapid biochemical remission, magnetic resonance imaging at 3 and 6 months revealed no tumor shrinkage despite increasing cabergoline to a maximal tolerated dose of 0.75 mg/week. The patient remained seizure free and resumed normal menses. This case illustrates a striking dissociation between hormonal sensitivity and a lack of radiological regression.