Hyperthyroidism is characterized by increased bone turnover and resorptive activity. Similar changes in remodeling are seen in osteoporosis. To study the pathogenetic role of thyroid hormone in osteoporosis, we measured concentrations of free and total thyroid hormones and investigated the sensitivity of the skeleton toward thyroid hormones in 14 osteoporotic, 16 estrogen-treated, and 15 normal postmenopausal women with comparable thyroid status. Triiodothyronine (T 3 , 60 g/day for 7 days) was administered to the three groups. The skeletal response was assessed by monitoring bone alkaline phosphatase (