Factor VIII activity and thyroid function.

Plasma factor VIII coagulant activity is decreased in hypothyroid patients and increased in hyperthyroid patients. We studied 21 untreated hypothyroid patients. Factor VIII coagulant activity was mildly decreased in association with significant depression of factor-VIII-related antigen and ristocetin cofactor activity in five patients. Factor-VIII-related properties significantly increased with oral thyroid replacement therapy in seven of 10 patients. Twenty-two untreated hyperthyroid patients were similarly evaluated. In 21 of these patients significant increases were noted in factor VIII coagulant activity, factor-VIII-related antigens, and ristocetin cofactor activity. Elevated factor-VIII-related properties returned to normal in all of 10 patients treated with radioactive iodine or propylthiouracil. We discuss the relation between thyroid function and factor-VIII-related properties in both hypothyroid and hyperthyroid patients.