Growth performance, health status, and the radiated thermal temperatures of groups of piglets consuming non-medicated feed and in-feed antibiotics (chlortetracycline hydrochloride and tiamulin) were monitored continuously for up to 35 d, from weaning at 28 d of age until moving to commercial pens at approximately 63 d of age. There was a trend (P = 0.1) for body weight at the time of moving to be higher for the medicated animals. The average daily gain was higher in medicated piglets (P = 0.03). Radiated temperature variables were the maximum (T_max) and the average (T_mean) pig temperatures, and both were higher for the medicated piglets (P < 0.0001). A threshold temperature was derived to partition the image and derive the background radiated temperature that was used to obtain the expected radiated temperature of the pigs. Temperature data were adjusted for environmental effects by expressing the residual temperature. The residual for T_max (T_maxRes) and for T_mean (T_meanRes) were higher in medicated piglets (P < 0.0001). In-feed antibiotics were associated with increased growth rates and with an increase in radiated temperature variables, particularly those corrected for environmental conditions, possibly mediated through increased metabolic activity related to growth.