The most poultry production in the world is dedicated to dry and tropical regions that these areas have long days with high temperature over 30⁰C. In these regions the most important problem is heat stress which could cause less production and more mortality in the flock. In this study the useful effects of early age thermal conditioning and feed restriction to improve performance of broiler chicks under summer heat stress were investigated. Three hundred chicks (cobb 500) were randomly divided into 4 treatments, with 5 replicates (15 chicks) for each treatments in completely randomized block design. Treatments include thermal conditioning group (TC), feed restriction group (FR), feed restriction-thermal conditioning group (TCFR) and control group (C). The groups TC and TCFR were exposed to thermal conditioning (38±2 C, for 24 h) at third day and the FR and TCFR groups were exposed to feed restriction for seven hours (10 am to 5 pm) at 7-14 days. Thyroid hormones were measured at 26^th and 40^th days. Results showed that, thermal conditioning group improved weight gain in comparison with control (p<0.05). Although, All the mentioned treatments had not significant effect on feed intake and feed conversion ratio. Thermal conditioning group increased concentration of  T3 and T3/T4 ratio at 26 day of age (p<0.05). The concentration of T3 and T3/T4 ratio at 26 day of age decreased and  T3 concentration increased at 40 day of age in feed restriction group, (p<0.05). Therefore, thermal conditioning at 3 day of age, improved performance of broiler chickens through changes in thyroid hormone metabolism in the whole breeding period.