Background: The thyroid gland secrete the hormone thyroxin (T4) and triiodothyronine (T3). They have a profound effect on the metabolism of hormones. The aim of this study was to servey the effect of the extract of plant root barberry on the amount of thyroid hormone in rat having hypercholesterolemia. Materials and Methods: In this experimental study, 40 Wistar rats were divided into 5 groups (n=8). Control group with normal diet, fat diet group and other groups, the experimental group received the same diet plus fat hydroalcoholic root extract of barberry maximum dose (300), The mean dose of (150), and a minimum dose of (75 mg/kgw) intraperitoneally injection were divided. After the end of this period (21 days), blood sampling and measurement of sample information were analyzed by using the t and Tukey test, and using of SPSS software version 11.5. Results: The results of statistical tests in the receiving groups extract reflects an increased amount of thyroid hormones T3 and T4 (P<0.05). The level of TSH in groups receiving the extract did not show significant changes Conclusion: Increases in T3 and T4 levels with no changes in TSH concentration indicate euthyroid hyperthyroxinemia probably.