Hypothyroidism has been associated with atherosclerosis. The mechanisms of atherosclerosis in patients with thyroid failure remain controversial. Hypofibrinolysis might be a risk factor for thromboembolic disease in subclinical hypothyroidism (SH). We measured fibrinolytic activity in patients with SH before and after levothyroxine (LT(4)) treatment and compared it to those of controls. We prospectively included 35 patients with SH and 30 healthy controls. We treated patients with LT(4) until almost 6 months after the euthyroid state has been achieved. We measured fibrinogen, D-dimer, antithrombin III (ATIII), plasminogen activator inhibitor-1 (PAI-1), tissue plasminogen activator (tPA) activity, and factor VII. Clinical and anthropometric variables were recorded for both groups. We found increased levels of fibrinogen, PAI-1, and factor VII and decreased levels of ATIII activity in patients compared to control (p < 0.001 and p < 0.05). Decrease of tPA was not significant (p > 0.05). At the end of the LT(4) treatment, significant decreases were determined in PAI-1 and factor VII (p < 0.05). In conclusion, our data suggest an important role of hypofibrinolytic and hypercoagulable state on the development of atherosclerosis in patients with SH and beneficial effects of LT(4 )treatment for decreasing the risk of atherosclerosis.