Description: Various methods are used to determine plasma thyroid hormones concentrations in medical diagnostic laboratories; but, some of them influence the results for thyroid hormone levels in domestic animals. The aim of this study was to compare the levels of two predominant plasma thyroid hormones using electrochemiluminescence immunoassay (ECLIA) and radioimmunoassay (RIA) methods in sheep. Blood samples were collected from the jugular vein of 30 clinically healthy and non-pregnant adult ewes. The separated plasma was analyzed to determine thyroxine (T4) and triiodothyronine (T3) concentrations. Results indicated significant differences in the T4 (P<0.05) and T3 (P<0.001) concentrations between the two methods, and only the level of T4 was higher when using the RIA method. The linear regression analysis of these hormones showed that the RIA and ECLIA results were significantly correlated: P=0.001, r=0.759 for T4; P=0.008, r=0.638 for T3. The ECLIA method exhibiнted a good precision for determining plasma T4 and T3 concentrations in sheep. It was concluded that ECLIA can be used as an alternative to RIA for assaying sheep T4 and T3 in clinical laboratories, but it has some limitations for application in veterinary diagnostic laboratories