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Brown adipose tissue (BAT) is a fat tissue present in most mammals, which is specialized in non-shivering thermogenesis. Thermogenic activity in BAT is commonly detected by PET/CT imaging with 18F-FDG, a glucose analog, in both rodents and humans. Increased uptake of the 18F-FDG within regions of BAT is interpreted as an indicator of increased thermogenic activity within the tissue. Despite its prevalence, recent studies have begun to question the use of 18F-FDG PET/CT for the detection of BAT thermogenesis, casting doubt on the direct relationship between glucose uptake and the degree of thermogenic activity. To this end, this study aims to compare 18F-FDG uptake within brown fat of mice that have very different BAT thermogenic capacity: wild type mice with functional BAT and mice lacking uncoupling protein 1– a protein in which brown fat thermogenesis is believed to be contingent upon. For this study, an imaging protocol was optimized to minimize variability in BAT stimulation. The data showed that 18F-FDG uptake in mice is highly variable, even under well-controlled experimental conditions. These results clearly indicate that, in general, the magnitude of 18F-FDG uptake in brown fat cannot be used as a surrogate measure for brown adipose tissue thermogenic activity