Antithrombin III deficiency (abbreviated ATIII deficiency) is a deficiency of antithrombin III. It is a rare hereditary disorder that generally comes to light when a patient suffers recurrent venous thrombosis and pulmonary embolism, and repetitive intrauterine fetal death (IUFD). Inheritance is usually autosomal dominant, though a few recessive cases have been noted.
The disorder was first described by Egeberg in 1965.
The patients are treated with anticoagulants or, more rarely, with antithrombin concentrate.
In kidney failure, especially nephrotic syndrome, antithrombin is lost in the urine, leading to a higher activity of Factor II and Factor X and in increased tendency to thrombosis.
Heparin enhances ATIII activity and neutralizes "activated serine protease coagulation factors." Patients with ATIII deficiency requiring anticoagulant therapy with heparin will need higher doses of heparin. ATIII binds to thrombin and then forms the thrombin-anti thrombin complex or TAT complex. This is a major natural pathway of anticoagulation. This binding of thrombin to AT is greatly enhanced in the presence of heparin. Heparin does not affect vitamin K epoxide, an enzyme required for the reduction of vitamin K, so giving vitamin K1 (Phytonadione) will not reverse the effects of heparin. 
Heparin is used in bridge therapy when initiating a patient on warfarin when in a hospital setting. It can be used in DVT prophylaxis and treatment, acute coronary syndromes, and ST-segment elevated MI.