Aim: Our aim was to investigate the changes in thyroid volume, thyroid hormone and autoantibody levels after 2 years in newly diagnosed euthyroid women with Hashimoto's thyroiditis (HT) and women with HT treated with levothyroxine (L-T4). Material and Method: We enrolled 60 women with HT (mean age 39.6 +/- 10.8 years) in this study. 27 patients were newly diagnosed and euthyroid (euthyroid group) and L-T4 group (n=33) consisted of women with HT treated with levothyroxine. Thyroid ultrasonography was performed and free T3 (fT3), free T4 (fT4), TSH, anti-thyroglobulin autoantibody (anti-TG) and anti-thyroid peroxidase autoantibody (anti-TPO) levels were measured. Thyroid volume was calculated with ellipsoid formula: (Volume (ml) = Length (cm) x Width(cm) x Thickness (cm) x 1/6 (sic)). Results: Median thyroid volume increased in all subjects (8.6 (1.8-46) vs 10.8 (3.9-54.5) ml, p=0.008). While serum fT3 levels decreased (2.9 +/- 0.5 vs 2.5 +/- 0.3 pg/ml, p<0.001), there was not changes in fT4, TSH, anti-TG ve anti-TPO levels (p>0.05). The change in thyroid volume was negatively associated with thyroid volume at basal (r= - 0.290, p=0.027) and TSH (r=-0.271, p=0.039) and positively correlated with fT3 (r= 0.356, p=0.006). While thyroid volume increased in euthyroid group (9.8 (6.6-35.1) vs 12.5 (6.9-54.5) ml, p=0.006), there was not a change in L-T4 group (7.5 vs 8.3 ml, p=0.428). Discussion: Thyroid volume increased in euthyroid women with HT, however did not change in patients treated with levothyroxine in two years. While thyroid autoantibodies did not change in two groups, hypothyroidism developed in 7.4% of euthyroid subjects.