Testosterone is something you naturally assume all men have in abundance. But for some obese men who lack the hormone, it can act as a powerful slimming aid.
Scientists found that a testosterone treatment led men to lose around a fifth of their weight after six years. On average, their body weight fell from 20 stone (127kg) to 16 (102kg) and their waist size decreased by 4.7in (12cm).
Tests showed that the 46 participants were also healthier, with reduced blood sugar, levels of "bad" cholesterol, blood pressure and markers of liver disease.
Meanwhile levels of "good" cholesterol, or high-density lipoprotein, which protects arteries, increased by 16%.
All the men taking part in the study were hypogonadal, meaning that they had low natural levels of the male hormone testosterone.
They received testosterone replacement therapy consisting of injections of the hormone for up to six years.
The findings, based on recorded health data from Germany, were presented at the European Congress on Obesity in Sofia, Bulgaria.
A summary of the research from the authors, led by Dr Farid Saad, from Bayer Pharma in Berlin, concluded: "Testosterone replacement therapy in hypogonadal men with obesity grade II and III resulted in meaningful and sustained weight loss.
"These findings are important because obesity grade III is commonly considered a potential indication for obesity (bariatric) surgery; as is grade II in the presence of type 2 diabetes. Thus testosterone therapy could be considered an alternative to obesity surgery in these men."