Background Recruitment of brown adipose tissue is a promising strategy to treat obesity and Type 2 diabetes, but the physiological effects of a large amount of metabolically active brown adipose tissue in humans are unknown. Case report In the present paper, we report a case of massive brown adipose tissue infiltration of the visceral adipose tissue depot in a person with Type 2 diabetes with a catecholamine‐secreting paraganglioma. The patient was evaluated with 18F‐fludeoxyglucose positron emission tomography/computed tomography on three occasions: pre‐therapy, during α‐blockade and postoperatively. During surgery, biopsies of visceral and subcutaneous adipose tissue were obtained and evaluated for brown adipose tissue. At diagnosis, brown adipose tissue glucose uptake, assessed by 18F‐fludeoxyglucose‐positron emission tomography, was massively increased. 18F‐fludeoxyglucose uptake was confined to known locations for brown adipose tissue, with additional uptake in the visceral adipose tissue. As a result of increased thermogenesis, resting energy expenditure was doubled. After surgical removal of the tumour, antidiabetic medicine was no longer needed, despite an 8.2‐kg weight gain. Conclusion These results show that human visceral adipose tissue holds an unprecedented potential for brown adipogenic differentiation; however, a detrimental effect on glucose metabolism persisted despite massive brown adipose tissue activity, with a doubling of resting energy expenditure.