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To describe the relationship between testosterone levels and type 2 diabetes (T2D).Multiple epidemiological studies have shown that low testosterone levels are associated with and predict the future development of T2D and the metabolic syndrome. Although this relationship is confounded by the association of total testosterone with sex hormone-binding globulin, free testosterone remains associated with measures of insulin resistance and T2D in some, but not all studies. Although the link between low testosterone levels and insulin resistance is not solely a consequence of adiposity, current studies suggest that a substantial component is mediated through its association with body fat, in particular abdominal visceral adipose tissue. This testosterone–fat relationship is bi-directional, as both weight loss and testosterone therapy increase testosterone levels, reduce fat mass, and decrease insulin resistance.Low testosterone levels are very commonly found in men with T2D and are associated with aging and obesity. Whether testosterone treatment in men with T2D decreases insulin resistance above that attributable to its fat-reducing effect is currently unknown. Future studies should compare testosterone treatment with lifestyle changes (exercise and weight loss measures), and other insulin-sensitizing agents. Until further evidence is available, testosterone therapy outside clinical trials should be reserved for diabetic men with unequivocal hypogonadism.