Thyroid hormones and bone

Introduction There is immense current interest in the effects of thyroid hormones on bone. This is largely due to concern that patients on thyroxine replacement therapy are at increased risk of developing osteoporosis; this concern follows a number of reports describing reduced bone mineral density in this group of patients. The issue is, however, uncertain and the purpose of this review is (i) to summarize what is known about the effects of thyroid hormones on bone at both an experimental and clinical level and (ii) to try to reach a greater understanding of the problem and its implications for patient management. Bone biology Bone remodelling requires the tightly coupled actions of osteoclasts and osteoblasts. A normal bone remodelling cycle takes approximately 200 days. Each cycle begins with activation of cells which become osteoclasts and start resorbing bone. This phase lasts for about 50 days and is terminated