Our case involves a 53 year old woman. Three years ago, she was investigated because of normal hemoglobin levels despite very a low erythrocyte count, which was revealed during the preoperative evaluation for ovarian cyst operation. The Direct Coombs test was found to be positive against complement and negative against IgG. Cold agglutinin titer was 1/448 (+). Due to the polyclonal IgM increase, secondary cold agglutinin disease (CAD) was considered but no factor could be found that would lead to cold agglutinin disease. During the post-operative follow-up, cold agglutinin titers increased with fluctuations in the patient. Twenty-four months after transabdominal hysterectomy and bilateral salpingooopherectomy operation, diagnosis of Hashimoto disease was made upon detection of subclinical hypothyroidism. No case of Hashimoto disease associated with CAD caused by polyclonal IgM has been reported until the present time.