BACKGROUND In hypothyroid patients, the risk for cardiovascular disease is higher and ultrasonography (US) demonstrates that the carotid intima-media thickness (CIMT) is significantly increased. We hypothesized that L-thyroxine replacement therapy might be able to reverse the process associated with increase in CIMT in patients with primary hypothyroidism. METHODS In this study, a total of 43 females with primary hypothyroidism and 21 euthyroid females as control group were included. In hypothyroid patients, CIMT was measured using US and the measurement was repeated 6 months after euthyroidism was achieved with L-thyroxine replacement therapy. Biochemically, lipid profile, high sensitivity C-reactive protein (hs-CRP), plasminogen activator inhibitory-1 (PAI-1) and fibrinogen levels were measured. RESULTS It was found that in hypothyroid patients the value of CIMT was significantly higher than those in control group (0.534+/-0.08 mm vs. 0.443+/-0.05 mm, respectively; p<0.001). However, the value of CIMT decreased significantly in all but two patients after euthyroidism was achieved with L-thyroxine replacement therapy (0.534+/-0.08 mm and 0.465+/-0.06 mm, respectively; p<0.001). Moreover, there was a positive correlation between the CIMT value and all other parameters except patient age, including total cholesterol (r=0.437, p=0.003), low density lipoprotein (LDL) cholesterol (r=0.415, p=0.006), total cholesterol/high density lipoprotein (HDL) cholesterol ratio (r=0.391, p=0.01) basal levels. CONCLUSIONS This report demonstrates that in patients with primary hypothyroidism, in addition to values of total cholesterol, LDL cholesterol, and total cholesterol/HDL cholesterol ratio, the CIMT value was higher compared to healthy controls. Importantly, the value of CIMT, as well as the levels of lipid parameters, decreased to normal level after L-thyroxine replacement therapy. Furthermore, significant correlations were detected between the changes of CIMT and the changes of total cholesterol and LDL cholesterol respectively. Thus, it is suggested that an increased CIMT value may be an objective sign of accelerated atherosclerosis in patients with primary hypothroidism.