BACKGROUND: Thyroid disease and osteoporosis are common problems often managed by primary care physicians. Despite many studies, confusion still exists about the effect of thyroid hormone on skeletal health. PURPOSE: To review evidence on the effect of thyroid hormone (from hyperthyroidism, exogenous or endogenous suppression of thyroid-stimulating hormone [TSH], and thyroid hormone replacement therapy) on skeletal integrity. DATA SOURCES: A MEDLINE search of papers published between 1966 and 1997. DATA SELECTION: Cross-sectional studies, longitudinal studies, and meta-analyses that had appropriate control groups (patients matched for age, sex, and menopausal status), made comparisons with established databases, or defined thyroid state by TSH level or thyroid hormone dose were reviewed. DATA EXTRACTION AND SYNTHESIS: Data synthesis was not straightforward because of changes in doses and types of thyroid hormone preparations; changes in definitions of thyroid hormone replacement therapy and suppressive therapies; problems with study design; differences in skeletal sites assessed (hip, spine, forearm, or heel) and techniques used to measure bone mineral density; and inclusion of heterogenous and changing thyroid disease states. Overall, hyperthyroidism and use of thyroid hormone to suppress TSH because of thyroid cancer, goiters, or nodules seem to have an adverse effect on bone, especially in postmenopausal women; the largest effect is on cortical bone. Thyroid hormone replacement seems to have a minimal clinical effect on bone. CONCLUSION: Women with a history of hyperthyroidism or TSH suppression by thyroid hormone should have skeletal status assessed by bone mineral densitometry, preferably at a site containing cortical bone, such as the hip or forearm.