IgG4 is present in very low levels in children younger than 10 years of age, so IgG4 deficiencies are not usually diagnosed before age 10. IgG4 may be undetectable in the serum of many “normal” adult individuals, and therefore low IgG4 alone is insufficient evidence of an antibody deficiency disorder requiring Ig replacement. IgG4–related systemic disease (IgG4-RSD), also known as hyper-IgG4 disease and IgG4-related disease is a disease in which inflammatory cells cause fibrosis, the deposition of connective tissue, in one or more organs. The disease is so named because the antibody subtype IgG4 can be detected on tissue samples and often at elevated levels in the bloodstream. The association with IgG4 is a relatively recent finding, and the condition has been described under numerous other names in the past. Diseases such asautoimmune pancreatitis, retroperitoneal fibrosis, mediastinal fibrosis, Riedel's thyroiditis, Mikulicz's syndrome, Kuttner's tumor and inflammatory pseudotumor are now regarded as forms of IgG4-RSD.