Subclinical hypothyroidism (SH) is associated with hypercoagulability and hypofibrinolysis. The objective of this study was to assess the effect of L-thyroxine (L-T4) treatment and to evaluate changes in the hemostatic abnormalities of patients with varying severities of SH. We measured tissue plasminogen activator (t-PA), plasminogen activator inhibitor-1 (PAI-1), D-dimer (DDI), fibrinogen (FIB), platelet counts (PLT), mean platelet volume (MPV), platelet distribution width (PDW), activated partial thromboplastin time (APTT), and prothrombin time (PT) in 149 female subjects. The prospective study included 54 patients in the control group, 53 patients with 4.2 mu IU/mL<TSH<10 mu IU/mL (mild SH), and 42 patients with TSH=10 mu IU/mL (severe SH). All patients with a TSH value of =10 mu IU/mL were given L-T4 replacement therapy, titrated to normalize their TSH levels. There were significant differences in the triglyceride (TG), total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), and serum uric acid (UA) levels, and in the MPV, PDW, APTT, FIB, PAI-1, t-PA, and DDI (P<0.05) measurements among three groups. These data were further tested using the least significant difference method for multiple comparisons. Patients in the severe SH group showed higher FIB, PAI-1 and t-PA levels, lower DDI levels and shorter APTT compared with the control group, whereas the hemostatic parameters in the mild SH patients were not different compared with those of the control group. After 6 months of L-T4 treatment, a significant decrease in FIB, PAI-1 and t-PA levels and an increase in APTT and DDI were observed in the severe SH group. In conclusion, SH patients displayed a distinct pattern of alteration of hemostatic parameters that was dependent on the severity of the disease. Patients with TSH levels =10 mu IU/mL displayed hypercoagulability, which was reversed by 6 months of L-T4 treatment.