We report a case of cerebral venous thrombosis (CVT) due to tyrotoxicosis in a patient with methylene-tetrahydro-folate-reductase (MTHFR) gene polymorphism C677T, (genotype 677TT), in which discontinuation of intravenous heparin was followed by clinical and radiological worsening despite warfarin treatment. We discuss the relationship between CVT, MTHFR hyperthiroidism and anticoagulant therapy.