This study was conducted to investigate the effect of adding either brewer yeast as a probiotic, iodine or thyroxin with yeast as synbiotics to sea bream (Sparus aurata)fingerlings diets on; growth performance, feed utilization, body composition, biometric measurements indices and some serum constituents of sea bream. Acclimatized fish were randomly distributed in 12 glass aquaria and stocking density was 15 fish per aquarium with an average weight 1.9±0.1 g/fish as triplicate groups per treatment. The 1^st group was considered as a control which fed on the basal diet (C). Three tested diets are formulated by adding 2g yeast/kg diet (Y), 2g yeast+0.01g potassium iodide/kg diet (YI) and 2g yeast+0.05 g thyroxin /kg diet (YT4) respectively for second, third and fourth groups. In the present work, addition of yeast with thyroxin followed by yeast alone to sea bream (Sparus aurata) fingerlings diets significantly improve growth performance, feed utilization efficiency and the fish biochemical composition comparing with the control one. The lowest growth performance and feed utilization values were recorded in fish fed diet supplemented with iodine when compared with corresponding values in fish of control group. Results of biometric measurements showed that adding iodine to fish diet significantly reduced gut length. Adding either iodine or thyroxin significantly increased serum lipase and decreased amylase contents.