Low Antithrombin Can Contribute to Clotting Disorders

Antithrombin (AT) is a naturally occurring anticoagulant that stops the action of thrombin and other clotting factors when clot formation is no longer necessary. Heparin also increases the effect of antithrombin. Changes in the antithrombin gene may cause low levels of antithrombin or abnormal antithrombin protein activity.


It should be recognized that antithrombin levels are usually lower at birth and typically don’t reach ‘normal’ levels until approximately six months of age.

Several conditions can reduce antithrombin levels, including:

  • Liver and kidney disease

  • Complications during pregnancy or labor and delivery

  • Cancer

  • Malnutrition

  • Gastrointestinal problems

  • Oral contraceptives or other medications


There are two types of antithrombin deficiency:

  • Type I: caused by low levels of the normal antithrombin protein

  • Type 2: caused by an abnormally functioning antithrombin protein

Antithrombin deficiency is an inherited condition. Patients with both types of antithrombin deficiency are at risk of clotting in both arteries and veins. The number of people presenting with symptoms related to antithrombin deficiency is estimated to be one in 2,000-5,000 people. Antithrombin deficiency in people who don’t display symptoms may occur as frequently as one in 600 people.

In patients with a history of a blood clot, the incidence of antithrombin deficiency ranges from 0.5% to 4.9%. People with antithrombin deficiency who also have defects in the heparin-binding site have a severe clotting phenotype that occurs early in life and often involves blood clots forming in the arteries. An abnormally-functioning antithrombin protein can affect how antithrombin binds to heparin or neutralizes the effect of thrombin in the absence of heparin.