Low serum thyroxine due to anti-thyroxine antibody.

In one case of untreated Hashimoto's thyroiditis, the serum thyroxine (T4) value, obtained by radioimmunoassay (RIA) employing resin to separate bound and free T4, was significantly lower than that obtained by competitive protein binding analysis (CPBA). The discrepancy was found to be due to the presence of anti-thyroxine autoantibody in the serum. This phenomenon is considered to be of practical importance in interpreting the T4 value by RIA in cases with autoimmune thyroid diseases.