A 3,3',5-triiodothyronine autoantibody (IgG, lambda) in a case of Hashimoto's thyroiditis

A 19-year-old Japanese woman with Hashimoto's thyroiditis appeared to be hypothyroid by clinical and laboratory criteria. However, the serum levels of total T3 and free T3 as measured by solid-phase radioimmunoassay were not compatible with the clinical features and other laboratory data, therefore the presence of serum T3 autoantibodies was suspected. Gel filtration analysis (Sephacryl S-300) of acid-treated serum trace-labeled with 125I-T3 revealed binding to high and intermediate molecular fractions, indicating that the T3 binding autoantibody may have been IgM and IgG. The antibody, however, was identified as IgG class and lambda type by immunoprecipitation and immunoelectrophoresis/autoradiography. The T3 binding autoantibody did not show antigenic cross-reactivity with T4 as far as we know from experiments. The affinity and the binding capacity of the antibody for T3 were estimated 6.4 X 10(6) l/mol and 9.1 X 10(-9) mol/l, respectively.