Background: The growth of lambs is influenced by several nutritional, genetic, and metabolic endocrine factors. Lambs that are fed on a high-level nutrient-dense diet exhibit quicker growth than those on a less aliment-fed diet. In addition,3-5-3'-triiodothyronine (T3) and thyroxine (T4) are the key hormones that regulate nutrient utilization and partitioning.

Objective: This study investigates the thyroid activity in Awassi lambs in relation to the sex of the animal,season,and the type of pregnancy (single & twin) by the direct measurement of the serum concentration of T3 and T4 at various growth stages.

Method: This study was conducted by utilizing a total of 19 animals of two-weeks old Awassi lambs (single and twins).The lambs were weaned gradually at the end of the second week of age and fed concentrate and hay.Blood samples were then collected, which was subsequently followed by the measurement of serum T3 and T4 concentrations by utilizing radioimmunoassay (RIA).

Results: Our data reveal a significant (p<0.01) drop in T3 at 14 weeks of age, while serum T4 in females was significantly higher (p<0.05) as compared to males. We conclude that the sex of the animal and the type of pregnancy had no impact on the thyroid hormonal values.

Discussion and Conclusion: Our results revealed variations in the levels of TH due to the impact of several determinants,including sex of the animal, type of pregnancy, and age of the lambs. Our data revealed TH mostly declines during warmer temperatures, and remains high at colder temperatures. Our study suggests that an increase in the ambient temperature between April-May in Iraq may play a significant role in the hormonal fluctuation process in lambs. The decline in the T3 and T4 values reflect the depression in thyroid activity.

Keywords:Awassi Lambs, T3, T4.