Mosekilde, L.&Melsen, F. Morphometric and dynamic studies of bone changes in hypothyroidism. Acta path. microbiol. scand. Sect. A,86:56–62, 1978.Bone biopsies were performed after tetracycline double‐labelling by transfixing the right iliac crest in fourteen hypothyroid patients. The bone changes in cortical and trabecular bone were determined by simple measurement and point counting on decalcified and undecalcified stained sections and compared to normal controls and to sex and age matched hyperthyroid patients. The amount of trabecular bone and the cortical porosity were unchanged in the hypothyroid patients compared to normal controls, whereas the mean cortical thickness was increased. The amount of osteoid and the length of the osteoid seams were normal, whereas the mean width of the osteoid seams was decreased. The linear calcification rate in cancellous bone was decreased, as were the active calcification surfaces (tetracycline‐labelled) and the percentage of osteoid covered surfaces active in mineralization. The osteoclastic resorption surfaces were unchanged in trabecular bone, whereas the osteoclastic activity in cortical bone was decreased. The osteocytic osteolysis was normal. The bone changes in hypothyroidism were opposite to the changes in hyperthyroidism characterized by a very low bone turn‐over with a reduced osteoid apposition and bone mineralization rate, an inactive osteoclastic resorption in trabecular bone and a decreased osteoclastic resorption in cortic