Researchers followed a group of mostly older, overweight men receiving injections of the hormone for up to five years to treat erectile dysfunction and other symptoms associated with low testosterone.
Their findings were presented at the 19th European Congress on Obesity in Lyon, France.
Researcher Farid Saad, DVM, says the dramatic weight loss came as a surprise.
"This study was not performed for the purpose of promoting weight loss," he tells WebMD. "This was an incidental finding that was entirely unexpected."
Low Testosterone Common in Obese Men
A total of 214 men remained in the study for at least two years, and just over half of these men were followed for another three years or more.
All the study participants received a long-acting injected testosterone, with two injections given during the first six weeks of treatment followed by injections every three months as long as they remained in the study.
Men who were treated the longest lost the most weight and saw the biggest reductions in waist circumference and body mass index (BMI).
After five years of follow-up, the average weight loss was 35 pounds and the average waist circumference dropped from about 42 to 38 inches. Most men also saw improvements in triglyceride levels, blood pressure, blood sugar, and LDL cholesterol.
Saad is a researcher with Bayer Pharma AG, of Berlin, Germany. Bayer manufacturers the long-acting testosterone treatment the men received, which has not been approved for use in the U.S.
Saad says the men in the study may have lost weight on the testosterone therapy because they had more energy to exercise.
"This is a theory, and, certainly, more research is needed to confirm our findings," he tells WebMD.
Testosterone for Weight Loss: 'More Study Needed'
There are concerns that prolonged testosterone therapy could increase prostate cancer risk, but there was no evidence of this among the men in the study.
University of Buffalo endocrinologist Sandeep Dhindsa, MD, agrees that more study is needed to confirm the safety and usefulness of replacement therapy for weight loss in men with low testosterone.
"The weight loss reported in this study was much greater than that reported in other studies," he tells WebMD. "Given this outcome and the safety questions about this treatment, it is important to replicate these findings."
These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.