Hyperthyroidism has significant impact upon both bone turnover and body composition. The present study was designed to investigate whether there is a connection between changes in body composition and bone mineral content at female patients with perturbed thyroid function. Sixty-seven long standing (over 6 months) overt hyperthyroid women had significantly lower bone mineral content as expressed by the Z score measured by quantitative ultrasonography (−0.86+/−0.69 compared to −0.08+/−0.37 in the age- and BMI- matched euthyroid control group of 82 women, P=0.01) and a modified body composition (evaluated by the bioelectrical impedance technique), with lower body fat percentage (39+/−2% compared to 44+/−1.9% in controls, P=0.01). Bone mineral content of hyperthyroid women was significantly correlated to serum alkaline phosphatase (R2=0.545, P<0.001), but not to the percentage of body fat (R2=0.0069, NS). Body fat percentage was however a good predictor for the bone mineral content of control euthyroid women (R2=0.176, P=0.027). We conclude that loss of bone mass in hyperthyroid women is caused rather by an increase in bone turnover, under the direct action of thyroid hormones, than by a thyroid hormone-induced decrease of body fat mass.