BACKGROUND: There is controversy about whether thyroid hormone therapy may lead to osteoporosis, and less is known about the clinically more important end point of whether its use increases fracture risk. METHODS: We used a case-control study to examine the association between thyroid hormone use and hip fractures among older women in a large managed care organization in Northern California. The subjects were 501 women > or =65 years of age who were hospitalized for hip fractures and 533 age-matched controls without hip fractures. RESULTS: No difference in the ever use or duration of use of exogenous thyroid hormone was found between cases and controls (odds ratio [OR] 1.1, 95% confidence interval [CI] 0.8,1.6). Hip fracture was associated with evidence of visual impairment, prior use of steroids, and number of falls. CONCLUSIONS: In women > or =65 years, an independent effect of thyroid hormone use on the risk of hip fracture was not found. This finding is reassuring, given the large number of women on thyroid hormone therapy today.