Sinai laying hens at 22 or 42 weeks age were fed layer ration containing 0.00 (control), 40 , 60 or 80µg of Eltroxine  /kg diet. This treatment was  continued  for 16 weeks (8 weeks  of Elroxine addition and other 8 weeks without Eltroxine) within the summer and winter seasons. Results showed that , absolute body weight change (ABWC) was significantly (P<0.01) reduced  compared to the control during the Eltroxine addition at level of  80µg/kg diet, while it is significantly  (P<0.05) increased compared to control at the same level during cessation of Eltroxine treatment. ABWC was  significantly (P<0.01) decreased in the old compared to young  hens during the Eltroxine addition and the whole experimental period. ABWC was significantly increased in the winter than  summer season either during addition or cessation of Eltroxine and during the  whole experimental period. Feed consumption was significantly decreased, and feed conversion was significantly improved  by addition of Eltrorxin to diets of  old than  young hens, and in the winter than summer season . Egg number egg weight and egg mass were insignificantly affected  by Eltroxine addition, but after cessation of Eltroxine, egg number and egg mass were significantly (P<0.01) increased 5.6 and 5.03% for hens fed diets containing 40µg Eltroxine when compared to control, while, these traits were significantly decreased by 6.3 and 7.8%, respectively for hens fed diets with 80µg  Eltroxine. Egg number was insignificantly affected between the young and old age, egg weight and egg mass were significantly higher with the old than young  hens during addition of Eltroxine , when cessation  of Eltroxine egg  number and egg mass were significantly (P<0.01) lower with the old  than young hens but egg weight was vice versa. Egg number, egg weight and egg mass were significantly (P<0.01) higher during the winter than summer season either during addition or cessation of Eltroxine period. During addition of Eltroxine period means of yolk index and Haugh Units were  significantly  decreased by 5.9% and 7.1 % with the old than young hens , values of  shall thickness and Haugh Units were significantly  (P<0.01) higher by 7.3% and 8.5% within  the winter than summer season. Mean of yolk index was significantly  (P<0.01) lower when cessation of Eltroxine by 4% during the winter than summer season. Shell, yolk and albumen weight percentage insignificantly affected with the Eltroxine addition either  during addition or cessation of Eltroxine period. Shell and albumen weight percentage were significantly (P<0.01) increased with young  than old   hens while yolk percent was vice versa. Absolute chick weight was significantly (P<0.01)  decreased compared with the control group by -3.4% at level of 80µg Eltroxine. Fertility and Hatchability percent were  significantly  (P<0.01) increased  for young than old hens, but absolute chicks  weight for old hens were significantly (P<0.01) increased by 9.8% compared with the young hens. Fertility, hatchability and  relative chicks weight  were significantly increased during the winter compared with the summer seasons. General, the results indicated that 40mg Eltroxine /kg diet the optimal level  for Sinai laying hens