Hypothermia has been shown to provide a protective effect after renal injury in animal models However  there has been little clinical investigation of the renal effects of hypothermia Taking advantage of the opportunity offered by the POLAR multicentre randomised controlled trial of hypothermia in traumatic brain injury (TBI)  this substudy will investigate whether in patients with TBI: a) early and sustained hypothermia reduces the severity  incidence and duration of Acute Kidney Injury (AKI)  b) the release of biomarkers of kidney injury including NGAL is reduced by cooling compared to maintenance of normothermia  c) the release of biomarkers of kidney injury including NGAL will predict the severity  occurrence and duration of AKI in patients with TBI