Microsomal antibody is an antibody for the section of a microsome in thyroid cells. There are two methods of measurements of microsomal antibodies, the semiquantitative microtiter particle agglutination test(MCHA) and the high sensitive assay(RIA, ELISA) for the anti thyroid peroxidase(TPO-Ab). MCHA is broadly useful for the diagnosis of autoimmune thyroid diseases and TPO-Ab is mainly measured in the occult autoimmune thyroiditis. TPO-Ab is expensive to compare with MCHA, in Japan. We recommend that MCHA is used in the routine test and TPO-Ab should be measured in the cases of patients with thyroid goiter and MCHA negative.