High levels of plasma von Willebrand factor in hyperthyroidism.

Both von Willebrand factor antigen and activity tested as ristocetin cofactor, were found to be increased in the 20 patients with hyperthyroidism, while changes detected on the 10 hypothyroid patients did not significantly differ from the mean values recorded in 19 healthy control subjects. It is considered that the high levels of endothelia-derived plasma von Willebrand factor in hyperthyroidism is the result of a long-standing stimulation of vascular endothelial cells by either a direct effect of thyroid hormones or by an excessive adrenergic activity. The present findings represent an important limitation in interpreting raised levels of plasma von Willebrand factor as a marker of endothelial injury or in terms of a thrombotic tendency.