Pituitary involvement in granulomatosis with polyangiitis: a diagnostic and therapeutic challenge.

Granulomatosis with polyangiitis (GPA) is a systemic necrotizing vasculitis associated with antineutrophil cytoplasmic antibodies. Although pituitary involvement is rare, it represents a clinically relevant manifestation that may present as a sellar mass mimicking a pituitary neuroendocrine tumour, leading to diagnostic delay and inappropriate management. We report the case of a male patient who initially presented at 24 years of age with recurrent epistaxis and nasal crusting, with histopathological evidence of chronic granulomatous inflammation of undetermined aetiology. Four years later, he developed a headache, diplopia, bitemporal hemianopia, polyuria, and polydipsia. Magnetic resonance imaging revealed a pituitary mass with suprasellar extension. An endocrine evaluation demonstrated hypopituitarism and arginine vasopressin deficiency. The patient underwent transcranial resection under the presumptive diagnosis of a non-functioning pituitary neuroendocrine tumour; however, postoperative imaging showed lesion progression. Further investigation revealed positive C-antineutrophil cytoplasmic antibodies and systemic features consistent with GPA. Histopathological reassessment with immunohistochemistry supported the diagnosis of granulomatous hypophysitis in the context of GPA. Induction therapy with cyclophosphamide achieved initial disease control, but a pituitary relapse occurred during maintenance therapy with azathioprine, prompting treatment escalation to rituximab. Rituximab led to a reduction in the pituitary mass and sustained clinical stability over four years. Persistent hypopituitarism required long-term hormone replacement. Pituitary involvement in GPA is an uncommon but important diagnostic challenge and may closely mimic a pituitary neuroendocrine tumour. This case highlights the need for careful integration of systemic clinical features, endocrine findings, imaging, and histopathological reassessment when evaluating sellar masses. Immunosuppressive therapy with cyclophosphamide and rituximab was effective in controlling pituitary involvement, although permanent pituitary hormone deficiencies may persist despite disease control.