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Computed tomography (CT) can be performed in horses in a standing position. It is becoming an increasingly used diagnostic imaging modality, with the distal limb being one of the most commonly examined anatomical regions. Navicular syndrome is a common cause of forelimb lameness in sports horses. Navicular bone abnormalities have been reported in both lame and non-lame horses using different diagnostic imaging modalities. However, longitudinal changes in the thickness of the palmar cortex of the navicular bone measured on CT reconstructions have not been described. The main objective of this study was to document the palmar cortex thickness of the navicular bone and its change over time on CT reconstructions in non-lame Thoroughbred racehorses. Fifty-six forelimbs from 28 Thoroughbred racehorses were included. CT examinations of the distal forelimbs were performed four times at approximately six months’ intervals. The thickness of the palmar cortex was measured at five locations: proximally and distally on sagittal reconstructions, and laterally, midsagittally and medially on transverse reconstructions. Differences in palmar cortical thickness between examination time points were assessed using Kruskal–Wallis test, followed by post hoc Dunn’s tests with Bonferroni correction. Differences between the left and right forelimbs and between measurement locations were evaluated using Wilcoxon signed-rank test or paired t-test. Associations between palmar cortex thickness and age, sex, bodyweight, height at the withers, bodyweight-to-height ratio, examination time point, left versus right forelimb, and number of race starts were assessed using linear mixed-effects regression models. Reference ranges at each measurement points were established. The palmar cortex was significantly thicker medially than laterally (p=0.048) and distally than proximally (p<0.001). Medial measurements were positively associated with age (p=0.030), examination time point (p=0.044), bodyweight (p=0.006), bodyweight-to-height ratio (p<0.001), and number of race starts (p=0.041). Lateral measurements were positively associated with age (p=0.014), examination time point (p=0.021), bodyweight (p=0.045), bodyweight-to-height ratio (p=0.013), and number of race starts (p=0.004). At the proximal measurement site, the palmar cortex was significantly thicker in the left forelimb than in the right forelimb (p=0.015). The reference ranges established in this study will help accurate interpretation of CT images of clinical cases. Based on the results, the thickness of the palmar cortex at the medial and lateral measurement sites was positively associated with age, body size, and the time spent in training.