Background: Because of the importance of thyroid function and its effect on different organs, such as the heart, this study was aimed to evaluate the effect of levothyroxine on cardiac function in children with subclinical hypothyroidism. Methods: This quasi-experimental study was performed on children aged 4-12 years old with diagnosis of subclinical hypothyroidism in Amirkola Children's Hospital during 2018-2019. Cardiac functional parameters, including ejection fraction (EF), fractional shortening (FS), ratio of early filling velocity to early diastolic mitral annulus velocity (E/E'), myocardial performance index (MPI), left ventricular end-diastolic diameter (LVEDD), and left ventricular end-systolic diameter (LVESD), were measured by echocardiography at baseline and 6 months after levothyroxine treatment. Results: Out of the 30 subjects, 19 (63.3) were boys and 11 (36.7) were girls. The mean age was 6.60±2.13 years old. The mean EF index was 63.13±3.01 percent before treatment, which increased to 69.07±4.50 percent after treatment (p<0.001). Also, the mean FS was 31.83±1.62 percent before treatment, which improved to 35.10±1.13 percent after treatment (p<0.001). The mean MPI was 0.28±0.02 before treatment, which increased to 0.33±0.03 after treatment (p<0.001). On the other hand, no significant difference was found in the mean E/E' before and after treatment. The mean LVEDD decreased from 3.47±0.46 cm before treatment to 3.05±0.40 cm after treatment (p<0.001), whereas LVESD non-significantly decreased after treatment versus before treatment. Conclusion: The results showed that treatment with levothyroxine may improve cardiac functional parameters in children with subclinical hypothyroidism
