BACKGROUND: Changes in the third metacarpal (MC3) mid-diaphyseal dimensions were measured radiographically in young Thoroughbred racehorses through race training and found to be associated with a higher risk of shin soreness. However, it is still unclear whether such negative MC3 morphologies are set congenitally or through development. OBJECTIVE: To quantify MC3 bone dimensional changes through growth. STUDY DESIGN: Longitudinal. METHOD: Twenty privately owned Thoroughbred foals were radiographed bimonthly from birth to 14 months of age. Four linear MC3 parameters (proximodistal MC3 length, mid-diaphyseal dorsal and palmar cortical bone thickness, and dorsopalmar medullary width) were measured using ImageJ on lateromedial radiographs. Associations with age (growth), MC3 length, differences between left and right limbs, and between sexes were analysed separately using a linear mixed effects model. RESULTS: The dorsal cortical bone thickness showed a positive linear relationship with growth (0.4 MAIN LIMITATIONS: A limited number of foals were included with similar breeding. CONCLUSION: The mid-diaphyseal dorsal cortical bone thickness and proportions could be measured to monitor musculoskeletal development and identify foals at increased risk of shin soreness or those that are immature or otherwise need more time before the commencement of fast training.