OBJECTIVE: Patients with thyroid diseases have abnormalities of blood coagulation including an alteration of von Willebrand Factor (vWF) levels. As vWF plays an important role in primary haemostasis, 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, 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 thyroxine.

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-Ag levels were associated with increased baseline platelet plug formation in patients with hyperthyroidism as compared to controls [114s (95%CI:105-122s) vs. 130s (120-140s) P = 0.01]. After 8 weeks of therapy with thiamazole serum concentrations of thyroxine (T4) and vWF levels decreased to normal values (P < 0.01 vs. baseline), and CEPI-CT was prolonged as compared to baseline (P < 0.01). During therapy with thyroxine, vWF levels increased (P < 0.05 vs. baseline) and CEPI-CT was shortened as compared to 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 diesase associated with overt hypothyroidism.