ABSTRACT Objective To evaluate the changes in left atrium structure and function in patients with subclinical hypothyroidism before and after treatment by real-time three-dimensional echocardiography (RT-3DE) . Methods 31 patients with clinical diagnosis of subclinical hypothyroidism (SCH) and received levothyroxine therapy were selected as the case group: pre-treatment group and post-treatment group, 33 healthy controls. All patients with SCH were followed up with replacement therapy until the euthyroid status was achieved. Echocardiography was followed up three months after normal euthyroid status.Comparison of structural parameters and functional changes of left atrium before and after treatment. Left atrial parameters include: left atrial maximum volume (LAVmax), left atrial pre-systolic volume (LAVpre), and left atrial minimum volume (LAVmin). The left atrial total emptying index (LAEF), the left atrial passive emptying index (LApEF) and the left atrial active emptying index (LAaEF) were calculated. Strain parameters were also obtained: left atrium strain during reservoir (LASr), left atrium strain during conduit (LAScd), left atrium strain during contraction (LASct). Results (1) Compared with the control group and after treatment, LAVpre, LAVmin, and LAaEF increased before treatment (P <0.05), and LAEF and LApEF decreased (P <0.05); there was no significant difference in LAVmax between the three groups (P> 0.05). (2) There was no significant difference in LAVmax, LAVpre, LAVmin, LAEF, LApEF, LAaEF after treatment compared with the control group (P> 0.05). (3) Compared with the control group and after treatment, the LASr and LAScd decreased before treatment, and the increase in LASct was statistically significant (P <0.05). Compared with the control group, the LASr and LAScd decreased after treatment, and the LASct increased. All were statistically significant (P <0.05). (4) Pearson correlation analysis was used to detect the correlation between TSH levels and LAEF, LApEF, and LAaEF before treatment: we found there was no correlation between TSH levels and LAEF (r = 0.16, P = 0.39), negative correlation between LApEF (r = -0.44, P = 0.01), positive correlation with LAaEF (r = 0.45, P = 0.01). Conclusion This study mainly proved that RT-3DE technology can detect the left atrial volume and mechanical function damage in patients with SCH at an early stage, and the left atrial damage was significantly improved after receiving levothyroxine therapy .