Aim: This study aimed to compare the hematological and biochemical parameters of hypothyroid and hyperthyroid patients when they were first diagnosed.Material and Methods: This study is a retrospective cross-sectional study.Hyperthyroid patients were diagnosed with elevated serum levels of fT3 and/or fT4 but decreased TSH levels compared to reference ranges (fT4: 12.3-20.2pmol/L; fT3: 3.71-6.70pmol/L; and TSH, 0.30-3.94mIU/L).Hypothyroid patients were diagnosed with decreased serum levels of fT3 and/or fT4 but elevated serum levels of TSH.Results: A total of 727 patients, 349 (277F/72M) diagnosed with hypothyroidism and 378 (306F/72M) diagnosed with hyperthyroidism, were included in this study.The number of women was statistically significantly higher (p<0.01)than men in both hypothyroid (%79) and hyperthyroid (%80) patient groups.When we compared the hypothyroidism and hyperthyroidism groups, we found a statistically significant difference between groups in terms of TSH, fT3, fT4, MPV, PLT, urea, creatinine, LDL-C, TC, TG, and TG/HDL-C ratio.In this study, a significant increase (p<0.01) in serum levels of TG, TC and LDL-C was observed in hypothyroid patients compared to hyperthyroid patients.We also found a negative significant correlation between fT3 and LDL-C in patients with hyperthyroidism (r: -0.238, p<0.01).Moreover, vitamin B12 deficiency (B12 levels <175 pg\ml), was present in 8% of hypothyroid patients and 6.3% of hyperthyroid patients.Discussion: Our findings emphasize that it is important to follow up both hypothyroid and hyperthyroid patients in terms of lipid parameters.Especially in patients with hypothyroidism, lipid metabolism is highly affected.