Pre-registration: Causal associations between circulating regulatory T cell subsets and bone mineral density — a two-sample Mendelian randomization study

This is a pre-registration for a two-sample Mendelian randomization study examining the causal associations between circulating regulatory T cell (Treg) subsets and bone mineral density. Exposure: 731 circulating immune cell traits from Orrù et al. (2020), including regulatory T cell subsets. Primary outcome: estimated heel bone mineral density (eBMD), n = 426,824. Secondary outcomes: total body BMD, forearm BMD, femoral neck BMD, lumbar spine BMD, and osteoporosis (FinnGen). Analysis status: Data extraction and statistical analysis were completed prior to this deposit. This retrospective registration serves purely as a transparent timestamp to prevent post hoc hypothesizing. No results were altered or selectively reported based on this deposit. Note: this document describes the analysis design only and does not contain effect estimates or P values.