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The present study was conducted to study the effect of exogenous thyroxin hormone on thyroid gland development and to determine the concentration of this hormone in the tissues of eggs, which affects the growth performance of tilapia larvae. The eggs and larvae were exposed to different doses of thyroxin solution from 0.025 mg/l to 0.1 mg/L and immersed in that solution for 1 hr. The present results showed that the level of thyroxin (T4) was higher in the newly hatched eggs then gradually decreased to the lowest level 30±0.04 mg /g wet weight (wt) after 96 hrs of treatment. Both treated larvae and the control group were similar in their concentration of total thyroxin at the period ranging from 3–7 days of post-hatching. The level of thyroxin (T4) was gradually increased after 17 days of post-hatching and reached the highest value of 8.38±2.31 mg/g wet wt. This level decreased and reached the lowest value of 1.97±0.79 mg/g wet wt after 27 days of post-hatching. The histological evidence of thyroxin on the thyroid gland showed that the thyroid follicles of the treated fish with thyroxin have more developed than that of the untreated fish (control group). [ABSTRACT FROM AUTHOR]