Background <p>IgG4-Related Disease (IgG4-RD) is a fibroinflammatory disorder initially identified as autoimmune pancreatitis but subsequently shown in numerous other organs with common pathological hallmarks. Constrictive pericarditis is an unusual presentation of IgG4-RD.</p> Case report <p>This case report is about a 45-year-old previously healthy Slavic male patient with complicated course of IgG4-Related Disease (IgG4-RD) with pancreatic cyst, constrictive-effusive pericarditis potentially triggered by 3-month antecedent Coxsackie virus infection with hand foot and mouth syndrome, who had 495 cc pericardial fluid drained, bilateral pleural effusions with 1&#xa0;L fluid drained from the right side. He was treated with methylprednisolone oral up to 48&#xa0;mg daily. With each decrease in steroid dose (given maximum glucose on metabolic panel to 903) or holiday in steroid up to 6&#xa0;weeks, he experienced worsening of positional and exertional shortness of breath. Final diagnosis and resolution of symptoms were obtained after pericardiectomy showing abundant fibrosis and plasma cells staining for IgG4. Remission was maintained on rituximab infusion.</p> Conclusion <p>This case highlights difficulty in diagnosing and treating IgG4-RD. His initial IgG4 serum level was normal to only mildly elevated. Definitive diagnosis was via tissue biopsy which required open heart surgery to access. Prior to the diagnosis, he was treated with methylprednisolone which led to severely elevated glucose levels. Oral steroids could not stop the progression of constrictive pericarditis. Pericardiectomy and subsequent treatment with rituximab led to continued remission.</p>