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.