Recurrent deep venous thrombosis despite well conducted anticoagulant treatment is an uncommon, but possible, event. It has been hypothesized that overt hyperthyroidism may increase thromboembolic risk. We present the case of an elderly man with a recurrent episode of deep venous thrombosis during optimal oral vitamin K antagonist treatment, associated with a new diagnosis of overt hyperthyroidism, with no evidence of occult cancer and normal levels of antiphosholipid antibodies.