Objective: In the present study, it was aimed to investigate the concomitance of additional cardiac problems, mainly mitral valve prolapse, in adolescents and pediatric patients with Hashimoto’s thyroiditis, by screening autoimmune markers. Materials and methods: Fifty-seven euthyroid patients, who applied to the Pediatric Endocrinology clinic at our institution with marked symptoms of hypothyroidism at the time of diagnosis, and were diagnosed and treated for Hashimoto’s thyroiditis, were included in the present study. All patients were evaluated by performing non-organ specific autoantibodies which could be tested at our institution, thyroid ultrasonography, two-dimensional echocardiography, and 24-h holter monitorization. Results: Of the 57 cases with Hashimoto’s thyroiditis, 48 (84.2%) were female, and nine (15.8%) were male. In the echocardiographic evaluation, mitral valve problems were detected in 10 (17.5%) of all cases; mitral valve prolapse was diagnosed in eight (seven females and one male) cases, and mitral insufficiency was diagnosed in two female cases. First-degree atrioventricular block was observed in only two patients during 24-h holter monitorization. Different non-organ specific autoantibody positivity was distributed as antinuclear antibody in 15 (26.3%) cases, anticardiolipin IgG in two cases, anticardiolipin IgM in three cases, tissue transglutaminase IgA in one, glutamic acid decarboxylase in one, anti-insulin antibody in four cases, antiphospholipid IgG in one, and antiphospholipid IgM in one case. Conclusion: It should be underlined that patients with Hashimoto’s thyroiditis should to be followed up closely for mitral valve prolapse and accompanying autoimmune diseases.