Background: We sought to clarify the possible role of elastic properties of the ascending aorta in the development of cardiac disease associated with hypothyroidism (HT). Methods: A total of 37 patients with HT (age: 39.3 +/- 8.9years) and 29 control subjects were studied. AscendingAortic (Ao) diameter, Ao elastic indexes, strain (AoST), distensibility (AoD), stiffness index (AoSI), and pressure strain modulus were calculated from the echocardiographically derived Ao diameters. Myocardial performance index (MPI), E/A ratio, isovolumetric relaxation time (IVRT), deceleration time (DT) were measured by Doppler echocardiography to assess diastolic LV function. Patients were treated with levothyroxine and followed-up for 6 months. Thyroid function tests and echocardiographic measurement's were repeated at the end of the study. Results: AoD (cm(2) dyn(-1) 10(-3)) and AoST (%) were significantly lower (3.8 vs. 6.1; P < 0.001, 7.4 vs. 12.6, P < 0.001; respectively), whereas AoSI was higher in HT patients (6.2 vs. 3.3; P < 0.001). After treatment, AoD and AoST were increased (5.7; P < 0.001 and 11.8; P < 0. 001; respectively), whereas AoSI was decreased significantly (3.7; P < 0. 001). Also, early / late mitral peak velocity ratio (Emax/Amax) was significantly lower in HT patients (1.19 vs 1.34; P < 0.001), whereas MPI was higher (0.52 vs. 0.42; P < 0.001). MPI showed a strong correlation with aortic root indexes [AoST (r = -0.61/P < 0.001); AoD, (r = -0.57/P < 0.002); AoSI, (r = 0.531P < 0.005)] in the HT group. After 6 months of therapy, MPI significantly decreased P < 0.001) and EIA ratios were normalized (P < 0.001). Conclusions: Ao root functions have an important role on diastolic LV function. Levothyroxine replacement therapy can reverse all of these adverse effects of HT.