Estrogen-induced deficiency and decrease in antithrombin III activity in patients with prostatic cancer.

Plasma antithrombin III activity was studied in 22 patients with prostatic cancer who were on estrogen therapy. Normal plasma antithrombin III activity varies between 0.80 and 1.40 U. per ml. A loading dose of 15 mg. diethylstilbestrol daily resulted in a marked decrease in plasma antithrombin III activity (mean 0.24 U. per ml.). Patients with the lower end of normal range of pre-treatment plasma antithrombin III activity may suffer acquired antithrombin III deficiency and, thus, a concomitantly increased risk of thromboembolic complications as a result of estrogen treatment. Patients on maintenance therapy of approximately 1 mg. daily appear to have normal plasma antithrombin III levels. The results obtained suggest that plasma antithrombin III activity should be monitored before and during estrogen therapy in patients with prostatic cancer.