Background: Thyroid hormones impact cardiac function and structure. Atherosclerosis, heart failure, and cardiac arrhythmias (especially atrial fibrillation) may occur depending on the degree of thyroid dysfunction. Asymmetric dimethylarginine is the main endogenous inhibitor of nitric oxide synthase, and it may increase in cardiovascular pathologies. The carotid intima media thickness is a predictive parameter for cardiovascular diseases. Aim: This study assessed biventricular systolic and diastolic functions, the carotid intima media thickness, and asymmetric dimethylarginine levels in female adolescents with hypothyroidism. Methods: The 71 participants were divided into a hypothyroid group (n = 37) and a control group (n = 34). All participants underwent laboratory blood tests (asymmetric dimethyl arginine [ADMA], complete two-dimensional, pulse, and tissue Doppler echocardiography examinations) and carotid intima-media thickness measurements. Results: The hypothyroidism group had significantly higher total cholesterol (TC), low-density lipoprotein, glucose, homeostasis model assessment of insulin resistance, and ADMA values (0.47 ± 0.03 vs. 0.27 ± 0.03 μmol/L) than the control group. The control group's end-systolic and end-diastolic diameters of the left ventricle were larger, and the study group's ejection fraction was higher, but their ventricular global diastolic function and ventricular masses did not differ. The carotid intima-media thickness was significantly higher in the hypothyroidism group (0.39 ± 0.007 vs. 0.36 ± 0.003 mm). The hypothyroid group's global biventricular systolic and diastolic functions were normal, but the study group's ratio of the mitral peak early diastolic wave to the peak late diastolic wave (E/A) was significantly higher, while the control group had significantly higher tricuspid E/A ratios. The hypothyroid group showed a positive correlation of thyroid-stimulating hormone with TC, low-density lipoprotein cholesterol, and left ventricular ejection fraction, as well as a negative correlation with free thyroxine (r = 0.71, P = 0.01; r = 0.73, P = 0.01; r = 0.69, P = 0.01; and r = -0.58, P = 0.01, respectively). Conclusions: Hypothyroidism in female adolescents may alter their myocardial geometry and function. Significantly higher asymmetric dimethylarginine levels and carotid intima-media thickness in patients with Hashimoto's thyroiditis may reflect endothelial dysfunction.