Case Report: A rare case of IgG4-related disease initially manifesting with diarrhea and jaundice.

Background: IgG4-related disease (IgG4-RD) is a systemic immune-mediated fibrosing inflammatory disorder with multi-organ involvement. Due to highly variable and nonspecific clinical manifestations, atypical cases are prone to misdiagnosis and missed diagnosis. This study reports an IgG4-RD case presenting with rare initial symptoms of diarrhea and jaundice, aiming to enrich the clinical spectrum of atypical IgG4-RD and improve early differential diagnosis. Case presentation: A 62-year-old male with well-controlled type 2 diabetes mellitus presented with 10 days of refractory mucoid diarrhea with occasional hematochezia and 2 days of progressive obstructive jaundice, pale stool, and generalized pruritus, accompanied by a 25 kg unintentional weight loss within one year. Laboratory tests showed markedly elevated transaminases, bilirubin, bile acid, alkaline phosphatase, gamma-glutamyl transpeptidase, CA19-9, erythrocyte sedimentation rate, and serum IgG4. Imaging revealed extrahepatic bile duct wall thickening and stenosis, sausage-like pancreatic swelling, and multi-organ lesions involving the bilateral renal hilum and retroperitoneum. Since pathological biopsy was declined by the patient and his family, a clinical suspicion of IgG4-related sclerosing cholangitis was established based on consistent clinical, serological, and typical multi-organ imaging features, although histopathology remains the gold standard for definitive diagnosis. The patient received liver-protective, glucocorticoid anti-inflammatory, nutritional, and anti-osteoporosis treatments. His symptoms resolved rapidly with significantly improved liver function, and he was discharged in stable condition. Ten-month regular follow-up was completed after discharge; no disease recurrence or adverse reactions were observed during gradual steroid tapering and maintenance phase. Conclusion: IgG4-RD with initial diarrhea and jaundice is easily misdiagnosed as cholangiocarcinoma or primary sclerosing cholangitis. Markedly elevated serum IgG4 combined with characteristic multi-organ imaging changes can strongly support a clinical suspicion of IgG4-RD, but cannot independently establish a definitive diagnosis without histopathological evidence. The gold-standard definite diagnosis requires integrated fulfillment of clinical, serological, radiological and pathological criteria. Glucocorticoid therapy is effective for symptom relief and biochemical improvement in suspected cases. The patient maintained sustained remission during the 10-month follow-up without relapse, demonstrating favorable medium-term prognosis, while long-term serial monitoring is still essential to further reduce late recurrence risk and optimize long-term prognosis.