Background <p>Heart transplant recipients often face the clinical dilemma of coexisting bleeding and thrombotic risks postoperatively. Conventional anticoagulation monitoring parameters (e.g., platelet count, fibrinogen, D‑dimer, and anti‑Xa activity) frequently exhibit limitations in evaluating complex postoperative patients, including delayed response, poor sensitivity, and lack of clear target ranges. This case report describes the use of thrombin–antithrombin complex (TAT) to guide anticoagulation management for complex thrombotic events after heart transplantation and explores its potential value as a supplementary monitoring tool.</p> Case presentation <p>A 50‑year‑old male heart transplant recipient developed multiple venous thrombi in the extremities and cerebral venous sinuses during the early postoperative period. Conventional monitoring parameters (platelets, fibrinogen, D‑dimer, and anti‑Xa activity) failed to effectively warn of thrombus progression or guide dose adjustments. Thrombosis continued to progress despite increasing anticoagulant intensity. Introduction of TAT monitoring revealed that its dynamic changes closely paralleled thrombotic events. Persistently elevated TAT levels (&gt; 8 ng/mL) indicated inadequate anticoagulation. After adjusting anticoagulant intensity based on TAT levels to maintain them within a safe range (&lt; 8 ng/mL), thrombosis was controlled. Later, when anti‑Xa activity decreased due to infection, stable TAT levels provided evidence to maintain the original anticoagulation regimen, avoiding overtreatment.</p> Conclusions <p>This case suggests that TAT may serve as a useful adjunctive monitoring tool for hypercoagulable states after heart transplantation. Its dynamic changes may more sensitively reflect thrombotic risk and assist in clinical decision‑making, particularly when conventional parameters are contradictory or lack clear targets, thereby facilitating more individualized and precise anticoagulation management.</p>