Background: Hypothyroidism has a large number of adverse effects on the cardiovascular system such as impaired cardiac contractility. Left ventricular (LV) asynchrony is defined as loss of the simultaneous peak contraction of corresponding cardiac segments. Objective: To assess systolic asynchrony in patients with overt hypothyroidism. Methods: Asynchrony was evaluated in 31 patients with overt hypothyroidism and 26 controls. Clinical hypothyroidism was defined as serum thyroid-stimulating hormone (TSH) more than 4.2 mu IU/mL with reduced free T4 less than 1.10 ng/dL. All the patients and controls were subjected to an echocardiographic study including tissue synchronization imaging (TSI). The time to regional peak systolic velocity (Ts) in LV via the six-basal-six-mid-segmental model was measured on ejection phase TSI images, and four TSI parameters of systolic asynchrony were computed. LV asynchrony was described by these four TSI parameters. Results: The demographic characteristics and conventional echocardiographic parameters of both groups were similar (except total and LDL cholesterol, TSH, free T3, and free T4). All TSI parameters of LV asynchrony were prolonged in hypothyroid patients compared to controls. The standard deviation (SD) of the 12 LV segments Ts was (53.5 +/- 14.1 vs. 29.3 +/- 15.5, P < 0.0001); the maximal difference in Ts between any 2 of the 12 LV segments was (154.5 +/- 37.3 vs. 91.9 +/- 45.2, P < 0.0001); the SD of Ts of the 6 basal LV segments was (47.9 +/- 15.9 vs. 27.1 +/- 16.4, P < 0.0001); and the maximal difference in Ts between any 2 of the 6 basal LV segments was (118.4 +/- 37.9 vs. 69.3 +/- 39.0, P < 0.0001). The prevalence of LV asynchrony was significantly higher in patients with hypothyroidism compared with controls (83.9% vs. 26.9%, P < 0.0001). Conclusion: Patients with overt hypothyroidism show evidence of LV asynchrony by TSI. (ECHOCARDIOGRAPHY 2010;27:117-122).