Boosting testosterone can significantly benefit many men with type 2 diabetes, new research suggests.
Study participants with low levels of the male hormone saw a dramatic increase in insulin sensitivity after testosterone replacement therapy.
An inability to respond to insulin is one of the hallmarks of type 2 diabetes.
The same scientists showed in 2010 that low testosterone is closely associated with the disease in men.
A third of type 2 diabetes sufferers were found to have low levels of the hormone, whether or not they were obese.
In the new study that included 44 diabetic men lacking testosterone, injections of the hormone increased the body's response to insulin - as measured by the tissue-uptake of glucose - by 32 per cent.
Testosterone treatment also resulted in the men losing more than six pounds (2.72 kg) of body fat on average while increasing muscle mass by the same amount.
Genes involved in insulin signalling became more active.
"This is the first definitive evidence that testosterone is an insulin sensitiser and hence a metabolic hormone, said lead researcher Professor Paresh Dandona, from New York State University at Buffalo in the US.
"We hypothesised that testosterone may be an anti-inflammatory and insulin sensitising agent since it has been known for some time that testosterone reduces adiposity and increases skeletal muscle.
"Our previous work has shown that obesity is associated with oxidative stress and inflammation, and inflammatory mediators are known to interfere with insulin signalling."
The findings are published in the journal Diabetes Care.
The men were randomised either to receive a testosterone injection or dummy placebo treatment every week for 24 weeks.
Despite the improved insulin response, there was no change to a recognised blood marker of type 2 diabetes.
This may only be seen when longer term studies are carried out, Dandona added.