Thyroid hormones have a crucial role in metabolism and proliferation of blood cells. Thyroid dysfunction induces different effects on blood cells such as anemia, erythrocytosis leukopenia, thrombocytopenia, and in rare cases causes' pancytopenia. It also alter RBC indices include MCV, MCH, MCHC and RDW. Thus this study attempted to evaluate effect of hypo & hyperthyroidism on blood cell count and RBC indices.This study performed on 102 patients with hypothyroid (14.1 years), 84 with hyperthyroid (15.6 years) and 118 healthy individuals (15.2 years) as control group. Initially patients TSH level of patients was determined by ELISA method, and then according to TSH ranges (0.3-5.5µIU/mL) patients were divided into two Hyperthyroidism (TSH5.5µIU/mL) groups. Then, complete blood count was measured by cell counter. Finally, obtained results were analyzed by SPSS software.Analyzes of obtained data revealed statistically significant difference between two groups of patients in RBC count, MCH, MCHC, RDW, HB and HCT(P-value0.05).In case of patients with unknown hematological dysfunctions, must be evaluated for thyroid hormones.