Autoantibody against diiodinated tyrosine-gastrin in a patient with Graves' disease

We describe autoantibodies against iodinated gastrin in a patient with Graves' disease. Values for serum gastrin differed in this case, depending on which of two different radioimmunoassay (RIA) kits was used. RIA with the dextran-coated charcoal method for separation of free tracer gastrin gave a value less than 9.5 pmol/L, whereas the value by a RIA kit by the double-antibody method was 318 pmol/L. The patient's serum contained a binding protein for 125I-labeled gastrin, as detected by Sephadex G-200 column chromatography. The IgG fraction was responsible for the ability of serum to bind 125I-labeled gastrin. Interestingly, of the two possible forms of iodinated gastrins, monoiodinated (MIT) and diiodinated (DIT) tyrosine-125I-labeled gastrin, only the latter bound to patient's IgG. Furthermore, DIT-gastrin, but not gastrin or MIT-gastrin, inhibited the binding of DIT-125I-labeled gastrin. The patient's serum evidently contains autoantibodies against DIT-gastrin that interfere with RIA of gastrin.