Uncontrolled Graves' Disease as A Driver of Amplified Warfarin Sensitivity.

Background: Thyroid function abnormalities can alter the pharmacokinetics of various drugs, including warfarin. Although a few case reports in the literature have highlighted this phenomenon, ours is the first in the region. Case description: We report a case of a 40-year-old female known to have uncontrolled Graves' disease due to non-compliance with medication and follow-up, with a history of mitral valve replacement, on warfarin 3 mg daily. She presented with a headache and was found to have a subdural haematoma and an INR of 10.58. It was observed that as she was approaching euthyroid state with antithyroid treatment; her warfarin requirement slowly increased to maintain the target INR. Conclusion: Our case highlights the relationship between hyperthyroidism and exaggerated response to vitamin K antagonists. Literature suggests there is an amplified response to warfarin in a hyperthyroid state. Therefore, a timely and close follow-up of INR and thyroid function test is warranted in patients with hyperthyroidism on anti-coagulation with warfarin. LEARNING POINTS: Uncontrolled hyperthyroidism can significantly increase warfarin sensitivity, leading to unpredictable and potentially dangerous INR elevations even on low doses.Improvement in thyroid function decreases warfarin sensitivity, often necessitating careful dose adjustment and frequent INR measurement to maintain therapeutic anticoagulation and prevent thrombotic and bleeding complications.Effective management of anti-coagulation in patients with thyroid dysfunction and mechanical heart valves requires multidisciplinary collaboration and patient adherence to optimise outcomes.