This study aimed to evaluate the dietary effect of the thiamine diphosphate (TD) glycogenic cofactor on the productive performance of male (M) and ewe (E) lambs exposed to outdoor conditions natural conditions of high summer temperatures in an arid region. Twenty 4month-old Dorper x Katahdin sheep were used, which were assigned to the following treatment combinations (n=5) according to a 2 2 -factorial arrangement under a randomized complete block design: 1) M without TD, 2) M with TD, 3) H without TD and 4) H with TD. No productive performance variable was affected (P>0.05) by the interaction or the DT main effect. M-lambs had higher (P>0.05) growth rate, feed intake and final weight than E-lambs. It is concluded that, regardless of the sex, DT does not improve productive performance of feedlot hair sheep under heat stress.