IgG4-related disease (IgG4-RD) is a chronic inflammatory and fibrotic condition that may affect multiple organs, including the paranasal sinuses. Diagnosis is difficult because the condition can resemble malignancies, infections, or autoimmune disorders. It often involves the pancreas, bile ducts, salivary glands, orbits, and lymph nodes, while sinus involvement is rare.

This case report describes a 61-year-old patient with chronic sinusitis and ocular symptoms, initially suspected as Graves’ disease. Subsequent imaging and laboratory tests ruled out thyroid-related orbitopathy, raising suspicion for IgG4-RD. The patient showed significant inflammatory infiltration in the paranasal sinuses, with bone destruction and involvement of extraocular muscles.

Histopathological analysis confirmed the diagnosis by demonstrating IgG4-positive plasma cells and elevated serum IgG4 (6.610 g/l) and total serum IgG (20.2 g/l). Glucocorticoids led to rapid clinical improvement. This case highlights the importance of multidisciplinary evaluation and increased awareness of IgG4-RD in atypical sinusitis cases presenting with ocular features.