The aim of this study was to evaluate the effect of thyroid functional state on cortical and trabecular bone mineral density (BMD) (g/cm2) in premenopausal and postmenopausal women. Control subjects were used as a reference population to calculate Z-scores from patient data. In patients with active hyperthyroidism, BMD was reduced in lumbar spine (LS; P < 0.01), femoral neck (FN; P < 0.01) and Ward triangle (WT; P < 0.0001) in comparison with reference standards. In premenopausal women treated in the past for hyperthyroidism or treated at the time of study with L-thyroxine in non-TSH-suppressive doses, there was no significant decrease in BMD. In postmenopausal women with hyperthyroidism in remission, we found a significant decrease in BMD in LS (P < 0.01), FN (P < 0.05) and WT (P < 0.0001). In postmenopausal women treated with L-thyroxine (L-T4), there was a significant decrease in BMD in LS (P < 0.01) and WT (P < 0.01). There was a significant negative correlation between the cumulative dose of L-T4 and BMD in FN (r = -0.688, P < 0.05) and WT (r = -0.657, P < 0.05) in postmenopausal women. Our findings suggest that the deleterious effects of thyroid hormones on BMD are accentuated in areas consisting predominantly of trabecular bone, e.g., the LS and WT. Postmenopausal women subjected to excess thyroid hormone represent a population at greater risk for osteoporosis.