The first Therapeutic Heparinization and Antithrombin Normalization Observational Study (THANOS-1) will determine the prevalence of antithrombin deficiency in patients with acute PE. Since many patients with PE are treated with heparin, which must bind to antithrombin to inhibit thrombin formation, antithrombin deficiency may affect the ability to achieve therapeutic anticoagulation. Understanding this will help us ensure that patients with PE are effectively treated and may lead to new adjunctive therapies.