Bone turnover is increased in favor of resorption in hyperthyroid patients. We aimed to examine whether osteoprotegerin (OPG), which has an inhibitory effect on osteoclasts, is correlated with any biomarkers for bone turnover in Graves' disease. Twenty-one patients with Graves' disease were examined in this study, before and after treatment. Briefly, OPG, calcium, phosphorus, PTH, free T3, free T4, TSH, TSH receptor antibody and TSH-stimulating antibody were measured. Elevated serum OPG levels were decreased in accordance with anti-thyroid treatment. This change of OPG level was associated with thyroid hormone free T4 (r = 0.175, p = 0.038) but not with free T3 (r = 0.164, p = 0.052) and TSH (r = 0.046, p = 0.59). Additionally, there was a negative correlation between OPG and PTH (r = -0.37, p = 0.0001). In stepwise regression analysis, the change in serum OPG levels during anti-thyroid treatment was significantly and independently associated with PTH (F ratio = 24.4, p < 0.0001). Our findings suggest that OPG may prevent excessive bone loss in the hyperthyroid state in accordance with the change of biomarkers for bone turnover.