We will study scanning database including data from July 2006 to June 2010 in Shanghai. BAT activity will be measured via 18F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT). Age, imaging date, sex, and outdoor temperature will be collected for all patients. To evaluate the effects of multiple metabolic parameters on BAT activity, we will collect height, body weight, fasting plasma glucose (FPG), medical history, diagnosis, smoking, alcohol consumption, exercise, liver triglyceride content, and thyroid metabolic activity for all patients with detectable BAT and their negative controls, consisting of two patients who underwent PET/CT scans at the same day but without detectable BAT.