Blood biochemical and hematology analyses are helpful indicators of the physiologic health of animals, particularly when making conservation and management decisions for threatened species. In this study, we 1) established blood biochemical reference intervals for two populations of threatened, free-ranging ornate box turtles (Terrapene ornata) in northern Illinois during their active season and 2) examined the effects of individual carapace temperature (T_c) on blood biochemical variables by using a Bayesian hierarchical framework. Individual blood variables differed throughout the active season (May–September 2015), but there were few distinct patterns in concentrations over time. When controlling for individual variability, blood biochemical variables potassium, sodium, chloride, ionized calcium, hematocrit (percentage of packed cell volume), and osmolality showed no effect of T_c (i.e., slope estimates for these variables were not credibly different from zero) and had little individual variation. Glucose and urea nitrogen were found to have slopes credibly different from zero, with glucose having an estimated positive slope and urea nitrogen having an estimated negative slope, suggesting different relationships in response to T_c when controlling for individual variability. These physiologic blood data will serve as important baseline reference values for the clinical evaluation of wild ornate box turtles presented for veterinary care or for comparison to other studies of wild populations. Further, this study highlights the importance of considering individual-level effects (e.g., T_c) on physiologic health variables.