Abstract Background Hashimoto thyroiditis (HT) is the most common cause of thyroid diseases in children and adolescents. Ventricular myocardial dysfunction associated with HT has been suggested to be related to the chronic inflammatory state associated with autoimmunity as well as to the endocrine effects. Objective To evaluate using speckle tracking echocardiography (STE) and estimation of carotid intima-media thickness (CIMT) in early detection of myocardial dysfunction in children with Hashimoto thyroiditis. Patients and Methods A cross-sectional case-control study was performed that included 15 patients with HT recruited from the pediatric endocrinology clinic, at Ain Shams University Hospital from October 2020 to July 2021. The patients were compared to 15 age and sex-matched healthy controls. Data were collected from patients’ files including age at initial diagnosis, initial level of thyroid hormones, antithyroglobulin antibodies, and antithyroid peroxidase antibodies. Results Mean age of enrolled patients was 13.20 ± 3.05 years. Cardiac functions in HT patients by using conventional Echocardiography and STE, both were correlated to thyroid hormones, auto- antibodies, lipid profile, and duration of disease (P < 0.05 for all). There were no statistically significant differences in conventional Echocardiographic parameters between patients and controls(P > 0.05 for all). Left ventricular global longitudinal strain (LVGLS) was within the normal range for both groups and showed no statistically significant differences between them (P > 0.05). Known HT patients had lower LVGLS mean (19.72 ± 1.22 %) than newly diagnosed patients (21.52 ± 0.99%, P = 0.014). There was no correlation between cardiac functions and the level of thyroid auto-antibodies (P > 0.05 for all). There was no statistically significant difference between patients and controls in CIMT (0.33 ± 0.05 vs 0.35± 0.05, P = 0.473). There was a positive correlation between the duration of disease and CIMT among patients (P-value= 0.025) Conclusion Patients with HT may have subclinical myocardial dysfunction which could be attributed to the chronic inflammatory state even with normal thyroid functions. STE can early detect the myocardial dysfunction on initial examination and follow up.