Thrombin is a serine protease, which is an enzyme that plays a key role in the coagulation cascade, where it helps to form fibrin clots by converting fibrinogen to fibrin. Thrombin is encoded by the F2 gene and is produced through the proteolytic cleavage of prothrombin. Thrombin has multifunctional roles, including acting as a procoagulant and anticoagulant. In its procoagulant role, thrombin activates platelets through its receptor on the platelets, and it regulates its own activity through feedback mechanisms. Thrombin also has chemotactic properties, enabling it to exert its effects during inflammation and vascular injury, and it has a mitogenic effect, stimulating the growth of mammalian cells, fibroblasts, and macrophage-like tumor cell lines. Thrombins role as an anticoagulant is mediated through binding to thrombomodulin, a receptor protein on the endothelial membrane of the blood vessel, initiating a series of reactions that leads to fibrinolysis.

Thrombin is used in medicine to stop bleeding during surgery, and it is also used as a topical thrombin to aid hemostasis whenever oozing blood and minor bleeding from capillaries and small venules is accessible and control of bleeding by standard surgical techniques (like suture, ligature, or cautery) is ineffective or impractical.

Thrombin time is a measure of how long the bloods plasma takes to form a clot, and it is one of several tests that check if the blood is clotting normally. A normal thrombin time is about 12 to 19 seconds, and a longer thrombin time can mean low fibrinogen, high fibrinogen, or fibrinogen thats not working normally.