Functional Suppression of a Prolactinoma by a Dopamine-Secreting Paraganglioma.

Prolactin-secreting pituitary adenomas are typically treated with dopamine agonists to inhibit prolactin secretion and reduce tumor size. Dopamine-secreting paragangliomas are rare neuroendocrine tumors of sympathetic and parasympathetic paraganglia and often do not provoke symptoms of catecholamine excess. Although overlapping genetic drivers have been described for paragangliomas and pituitary adenomas, biochemical crosstalk between coexisting tumors is underexplored. We describe the case of a 52-year-old male individual who presented with cerebrospinal fluid (CSF) rhinorrhea and was found to have an invasive, 4.2-cm pituitary mass with modestly elevated prolactin (130.9 ng/mL [130.9 µg/L], reference range [RR] 2-18 ng/mL [2-18 µg/L]). Additional imaging discovered a mediastinal mass suspicious for a thoracic paraganglioma. Biochemical screening demonstrated marked elevation of plasma and urinary dopamine. Following paraganglioma resection, dopamine levels normalized, but prolactin rose significantly (877.8 ng/mL [877.8 µg/L]), suggesting an endogenous dopamine agonist-like effect from the paraganglioma to suppress pituitary prolactin hypersecretion. Pituitary pathology was notable for a PIT1 (pituitary transcription factor-1)-lineage pituitary adenoma with absent immunohistochemical staining for prolactin. Genetic testing found a previously unreported germline