Blood coagulation, fibrinolytic activity and lipid profile in subclinical thyroid disease: subclinical hyperthyroidism increases plasma factor X activity
Authors: Erem Cihangir 
Published: February 2006

Background and objectives  Various abnormalities of coagulation and fibrinolysis occur in patients with thyroid diseases, and may range from subclinical laboratory abnormalities to clinically significant disorders of coagulation and, rarely, major haemorrhage or thromboembolism. The influence of subclinical hypothyroidism (SHypo) on haemostasis is controversial, both hypercoagulable and hypocoagulable states have been reported. A hypercoagulable state might be a risk factor for thromboembolic disease in SHypo. On the other hand, subclinical hyperthyroidism (SCHyper) is associated with enhanced cardiovascular risk. In the English literature, there are no studies on changes in coagulation and fibriolytic status in subjects with SCHyper. Therefore, the aim of the present study was to investigate the markers of endogenous coagulation and fibrinolysis, and to evaluate the relationships between serum lipid profile and thyroid hormones and these haemostatic parameters in subclinical thyroid patients.

Design and methods  Various haemostatic parameters were investigated in 30 patients with SHypo and 20 patients with SCHyper and compared to 20 euthyroid controls. Prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrinogen, factors V, VII, VIII, IX and X activities, vWF, antithrombin III (AT III), protein C, protein S, tissue plasminogen activator (t‐PA) and tissue plasminogen activator inhibitor‐1 (PAI‐1), as well as common lipid variables, were measured. The relationships between serum thyroid hormones and these haemostatic parameters were examined.

Results  Compared with the control subjects, only FX activity was significantly increased in patients with SCHyper (P < 0·01). Total cholesterol (TC) and low‐density lipoprotein cholesterol (LDL‐C) levels were significantly higher in patients with SHypo compared with the control group (P < 0·001 and P < 0·01, respectively). TC levels were significantly higher in patients with SCHyper than in controls (P < 0·05). No differences could be found in coagulation/fibrinolysis parameters between subclinical hypothyroid patients and control subjects.

 In patients with SCHyper, serum TSH level was positively correlated with FX activity (r: 0·58, P < 0·01) and inversely correlated with PAI‐1 (r: −0·55. P < 0·05). Serum TG levels were inversely correlated with plasma activities of factors V, VII, VIII, IX, X and vWF (r: −0·83, P < 0·001; r: −0·68, P < 0·05; r: −0·61, P < 0·05; r: −0·77, P < 0·01; r: −0·63, P < 0·05; r: −0·60, P < 0·05, respectively). Serum TC levels were positively correlated with plasma fibrinogen levels (r: 0·72, P < 0·05). Serum HDL‐C levels were positively correlated with protein S activity (r: 0·68, P < 0·05) and negatively correlated with F VII activity (r: −0·69, P < 0·05). Also, in patients with SHypo, serum TG levels were positively correlated with serum TSH levels (r: 0·42, P < 0·05), plasma activities of factors V, VII and X (r: 0·42, P < 0·05; r: 0·54, P < 0·01; r: 0·57, P < 0·01, respectively) and negatively correlated with plasma fibrinogen levels (r: −0·41, P < 0·05). Serum TC levels were positively correlated with factors V and X (r: 0·42, P < 0·05; r: 0·58, P < 0·01, respectively) and negatively correlated with t‐PA Ag levels (r: −0·44, P < 0·05). Serum HDL‐C levels were inversely correlated with F VII activity (r: −0·48, P < 0·05).

Interpretation and conclusions  Some differences were found in the haemostatic parameters and lipid profile between the subclinical thyroid patients and healthy controls. Increased factor X activity in patients with subclinical hyperthyroidism represent a potential hypercoagulable state, which might augment the already existing risk for atheroscleroic complications. Also, subclinical hypothyroid patients exhibit a more atherogenic lipid profile compared with healthy individuals. Therefore, subclinical hypothyroidism is also asociated with an increased risk of cardiovascular disease. However, thyroid hormones may play a role at different levels of the complex haemostatic system in subclinical thyroid disease.