Pituitary Corticotroph Adenoma in a Woman with Long-Standing Addison's Disease: A Histologic,immunocytochemical, Electron Microscopic,and In Situ Hybridization Study

A 64-year-old woman with long-standing Addison’s disease owing to destructive immune adrenalitis presented with hyperpigmentation and progressively increasing blood adrenocorticotrophic hormone (ACTH) levels. Magnetic resonance imaging demonstrated a pituitary microadenoma, which was removed by transsphenoidal surgery and investigated by histology, immunocytochemistry, transmission electron microscopy, andin situ hybridization (ISH). The morphologic studies revealed a basophilic, periodic acid-Schiff (PAS)-positive pituitary adenoma immunoreactive for ACTh and β-endorphin and in several cells for α-subunit. By transmission electron microscopy, the tumor was a densely granulated corticotroph adenoma, which, by ISH, expressed pro-opiomelanocortin (POMC) mRNA. The lack of corticotroph hyperplasia in the nontumorous adenohypophysis was an intriguing finding. Corticotroph adenomas in patients with long-standing Addison’s disease were very rarely examined by morphology. Our report includes a detailed morphologic analysis and is the first demonstration of POMC mRNA in the tumor cells using ISH. The question of whether the adenoma was related to increased secretory activity secondary to protracted hypocorticism or developed independently unrelated to deranged endocrine homeostasis remains unresolved. The lack of corticotroph hyperplasia in the nontumorous adenohypophysis favors the interpretation that hypothalamic stimulation played no major role in adenoma formation in our case.