Antithrombin III deficiency is a genetic disorder that causes the blood to clot abnormally.
Antithrombin III is a protein in the blood that blocks blood clots from forming. Congenital antithrombin III deficiency is a genetic disease. It occurs when a person receives one abnormal copy of a gene from a parent with the disease.
The abnormal gene leads to low levels of antithrombin III. These low levels of antithrombin III can cause abnormal blood clots (thrombi) that may damage organs.