Rationale: Immunoglobulin G4 related disease (IgG4-RD) rarely coexists with other autoimmune diseases, though we had a patient whose primary clinical problem was shifted from IgG4-RD to systemic lupus erythematosus (SLE) after gastrectomy. The present paper aimed to report pathological findings and clinical course of the patient. Patient concerns: The patient was a male aged 74 years old with gastric cancer characterized by the following symptoms: Raynaud phenomenon, polyarthralgia, and swollen parotid glands on both sides. Before gastrectomy, laboratory examination results showed renal dysfunction, hypocomplementemia, antinuclear antibodies (ANAs) positivity, and elevated serum IgG and IgG4 levels. Diagnosis: Based on postoperative renal biopsy showing severe plasma cell infiltration with tubulointerstitial fibrosclerosis, the patient was diagnosed with IgG4-RD. Despite significant improvement in renal function and reduction in parotid gland swelling during the postoperative follow-up period, after 7 months of the gastrectomy, anti-DNA antibody levels were increased and serositis was detected, which indicated the onset of SLE. IgG4-type ANA were also detected in the sera of the patient. Interventions: Treatment by oral prednisolone at 30 mg/day was initiated. Outcomes: Pericardial fluid, pleural effusions, and thickening of the gallbladder wall improved after 3 months of treatment according to computed tomography. Lessons: This study presented a rare case of comorbidity, wherein the patient’s primary problem progressed from IgG4-type ANA-positive IgG4-RD to SLE after excision of gastric cancer.
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