Increased bone, resorption previously found in hyperthyroidism might be caused by a direct stimulating effect of thyroid hormone(s) on bone cells or by an increased sensitivity to circulating parathyroid hormone. In order to disclose qualitative differences in the response of bone resorbing cells to excess parathyroid hormone and excess thyroid hormone(s), histomorphometric analysis of iliac crest biopsies was performed in 25 hyperparathyroid and 40 hyperthyroid patients after tetracycline double-labelling. The main target cells for parathyroid and thyroid hormones were different. Parathyroid hormone stimulated osteocytic osteolysis and increased osteoclastic resorption surfaces equally in trabecular and cortical bone. The osteoclastic resorption was inactive. Thyroid hormone(s) had no effect on osteocytes but increased the osteoclastic resorption surfaces in trabecular and cortical bone, with a pronounced preponderance in cortical bone. The osteoclastic resorption was active and followed by a significant loss of both cortical and trabecular bone. The findings support the assumption that increased bone resorption in hyperthyroidism is caused by a direct stimulating effect of thyroid hormone(s).