Relationship between thyroid hormones and fibrinogen levels.

Hypothyroid patients may have several haemostatic abnormalities such as modification of the coagulation proteins and a bleeding tendency. However, conflicting data have been reported, although in some reports, the groups of patients studied were small. We previously reported in a large population that plasma D-dimers and free thyroxine (FT4) levels were inversely correlated. Since D-dimers are also associated with fibrinogen levels, we analysed the relationship between fibrinogen and FT4 levels. The population consisted of 959 patients (61% men and 39% women), 28% of whom were current smokers. We showed that fibrinogen and FT4 levels were inversely correlated (r = -0.12, P = 0.0001) in the overall population, in the non-smoker subgroup (r = -0.13, P = 0.0007) and in the subgroup of patients with strictly normal FT4 levels (r = -0.11, P = 0.0006). The relationship between FT4 and fibrinogen levels was independent of age, sex and smoking habits. Furthermore, there was a significant difference between the mean fibrinogen levels in the patients in the lowest FT4 subgroup and those in the highest FT4 subgroup, which was similar to the difference previously observed in patients with or without evidence of atherosclerotic disease in epidemiological studies. In conclusion, we found a negative and independent relationship between fibrinogen and FT4. These results are consistent with our previous findings of a tendency to a more coagulable state in patients with normal-low FT4 levels compared with those with normal-high FT4 levels.