In this study, it was aimed to interpret the effects of hypothyroidism and hyperthyroidism on hematological parameters in rats. A total of 42 male Wistar rats were divided into 3 equally groups as euthyroid, hypothyroid, hyperthyroid. Hypothyroidism and hyperthyroidism were induced by 6-n-propyl-2-thiouracil (PTU; 0,05%, w/v) and L-thyroxine (L-T4; 0,0012%, w/v) received in drinking water, respectively. Blood samples were withdrawn from 7 rats of each group at the end of 3rd and 6th weeks for determination of the hematological parameters and serum hormone profiles. Among the 3rd week groups only percentage of monocyte was increased (p<0.05) in the hypothyroid group when compared to the euthyroid group. In the 6th week hypothyroid group, red blood cell (RBC) counts, hemoglobin (Hb) concentration, hematocrit (Hct) value and percentage (%) of red cell distribution width (RDW) were lower (p<0.05) than the 6th week euthyroid and hyperthyroid groups. However, plateletcrit (PCT) % was higher (p<0.05) in the 6th week hypothyroid group compared to the 6th week euthyroid group. In the 6th week hyperthyroid group, mean corpuscular hemoglobin concentration (MCHC) and platelet (PLT) counts were lower (p<0.05) than the 6th week hypothyroid group. In the 6th week hyperthyroid group percentage of lymphocyte was decreased (p<0.05) and percentage of granulocyte was increased (p<0.05) when compared to the 6th week euthyroid group. In conclusion, elongation of the treatment duration with PTU and L-T4 could lead to changes on some hematological parameters. However, these values remained within physiological ranges.