Objective: Patients with thyroid diseases have abnormalities of blood coagulation including an alteration of von Willebrand factor (vWF) levels. Because vWF plays an important role in primary hemostasis, we hypothesized that heightened and decreased vWF levels in hyper- and hypothyroidism enhance and decrease platelet plug formation, respectively.
Methods: We followed a cohort of 120 patients with overt hyperthyroidism, patients with subclinical and overt hypothyroidism, and euthyroid controls. vWF and in vitro platelet plug formation as collagen-epinephrine-induced closure time (CEPI-CT) were measured at baseline and during therapy with thiamazole or T(4).
Results: Baseline vWF levels were higher in patients with hyperthyroidism and lower in patients with overt hypothyroidism than in controls (P < 0.01). High vWF antigen levels were associated with increased baseline platelet plug formation in patients with hyperthyroidism as compared with controls [114 sec (95% confidence interval, 105-122 sec) vs. 130 sec (120-140 sec), P = 0.01]. After 8 wk of therapy with thiamazole, serum concentrations of T(4) and vWF levels decreased to normal values (P < 0.01 vs. baseline), and CEPI-CT was prolonged as compared with baseline (P < 0.01). During therapy with T(4), vWF levels increased (P < 0.05 vs. baseline) and CEPI-CT was shortened as compared with baseline (P < 0.01).
Conclusion: Hyperthyroidism-induced vWF elevation is associated with enhanced platelet function and therefore shortened CEPI-CT values. These changes may contribute to the higher risk for cardiovascular disease in patients with hyperthyroidism. Platelet plug formation decreases during therapy with thiamazole. Furthermore, CEPI-CT appears to be sensitive to detect acquired von Willebrand disease associated with overt hypothyroidism.