Granulomatosis with polyangiitis (GPA) and microscopic polyangiitis (MPA) are rare forms of antineutrophil cytoplasmic antibody-associated vasculitides, characterized by systemic inflammation and necrosis of small blood vessels, which can lead to multiorgan damage. GPA is identified by the presence of granulomas and serine proteinase-3 positivity, while MPA is marked by necrotizing vasculitis without granulomas and is associated with myeloperoxidase (MPO) positivity. Central nervous system involvement is more frequent in GPA, affecting 7% to 10% of patients, compared to its occurrence in approximately 1% of MPA cases. In this case report, we present a 41-year-old woman who exhibited symptoms of pituitary mass effect, initially suspected to be a macroadenoma. Further investigation revealed pituitary enlargement due to MPA, confirmed by positive MPO antibodies and lung biopsy findings consistent with MPA. After treatment with prednisone and rituximab, the patient's pituitary gland returned to its normal size, with significant improvement in her symptoms. This case highlights the diagnostic complexities in differentiating MPA from more common causes of pituitary enlargement and underscores the necessity of considering vasculitic origins in similar clinical scenarios. Further research is essential to deepen the understanding of the pathophysiology and to optimize the management of pituitary involvement in MPA.