PURPOSE: Dynamics of bone mineral density(BMD) and bone metabolism in children and adolescents with hyperthyroidism have not been thoroughly investigated. The aim of this study was to study how the improvement of thyroid function with antithyroid treatment influenced bone metabolism and BMD in children and adolescents with hyperthyroidism. METHODS: Serum levels of osteocalcin(OC), bone-specific alkaline phosphatase(b-ALP), and carboxyterminal telopeptide of type 1 collagen(ICTP) were measured just before, and six, and 12 months after commencement of methimazole treatment in 12 subjects(aged 4.4-15.6 years) with serial measurement of femoral and lumbar BMD. RESULTS: A significant increase in BMD and its SDS of the femur and lumbar spine was observed during the first six months(P<0.01; P<0.05) and the next six months of treatment(P<0.01; P<0.01). Serum levels of OC and b-ALP, and their SDSs before treatment were higher than the normal values. OC decreased significantly at 12 months(P<0.05), whereas b-ALP did not show any significant change during treatment. Serum levels of ICTP and its SDS before treatment were higher than normal values, but decreased into the normal range at six months of treatment(P<0.01), and remained stable during the next six months. CONCLUSION: This study demonstrates that hyperthyroidism increases bone turnover, decreases the relative rate of bone formation vs resorption, and decreases BMD in children and adolescents. These data also suggest that bone formation exceeds bone resorption within six months of antithyroid treatment and a significant increase of BMD occurs continuously during the first 12 months of treatment.