Bone mineral density (BMD) has been shown to decrease in Graves' disease (GD) as a result of the high rate of bone turnover with a predominance for resorption. As individual differences in BMD are great , and they change with age, it is difficult to compare levels in one patient with levels in others. This makes the clear evaluation of BMD levels in individuals impossible. For 3 female monozygotic twins from 3 families (A-1 and A-2 at 27 years old, B-1 and B-2 at 26 years old, C-1 and C-2 at 13

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... years old) , we analyzed the relationships between thyroid function and indices of bone metabolism, and also carried out a comparative study between BMD and bone turnover markers, with respect to age and medical treatment of patients with GD. The BMD values for the lumbar spine (L2-L4) and the proximal section of the femur (Hip) for A-1, who was in a state of subclinical hyperthyroidism, were higher than those for A-2, whose hyperthyroidism had not been treated. The BMD values for both sites in these twins were lower than those for healthy young Japanese women. Serum bone type alkaline phosphatase (B-ALP) and osteocalcin (OC) concentrations were high in both A-1 and A-2. Urinary deoxypyridinoline (DPD) was higher in A-2 than in A-1 . For B-2, who was taking Methimazole (MMI), the BMD values for both sites were lower than those for B-1, who had not yet developed GD . Serum B-ALP and OC concentrations were high in both B-1 and B-2. There was no difference in the BMD of C-1, who was taking MMI, and C-2, who had not yet developed GD. Serum B-ALP and OC concentrations were high for both C-1 and C-2. Our results suggest that 1) the decrease of BMD in GD is due to high-turnover type bone metabolism in which the functions of both osteoclasts and osteoblasts are accelerated ; 2) when GD occurs in adults , BMD does not recover completely, even if the patient has become euthyroid as a result of medical treatment; and 3) The decrease in BMD caused by GD in patients who are still growing can be minimized. Key words : Graves)disease, bone mineral density , bone metabolism, twin study Yoshio MORITA, et al phase.