Older men with lower levels of the male sex hormone testosterone in their blood may be more prone to depression, a study suggests.
A study of about 4,000 men aged over 70 found those with lowest testosterone were three times more likely to be depressed than those with the most.
Researchers suspect the hormone may affect levels of key brain chemicals.
The study, by the University of Western Australia, features in Archives of General Psychiatry.
Research has found that women are more likely to be depressed than men until the age of 65, when the difference between the genders almost disappears.
Testosterone levels decline with age - but there is wide variation.
The Australian team studied 3,987 men over the age of 70. Each gave blood samples and took part in tests to determine whether they were depressed.
In total 203 of the participants were assessed as being depressed.
They had significantly lower levels of both total testosterone, and free testosterone, which is not bound to proteins.
The researchers then adjusted the data to take account of factors such as educational attainment and body fat levels.
They found those men whose level of free testosterone was in the bottom 20% were three times more likely to be depressed than those in the top 20%.
The researchers said further work was required to confirm their findings.
But their work raised the possibility that treatment to boost testosterone levels in older men may be an effective way to treat depression.
Raised death risk
A previous study of 800 men over the age of 50 found that those with low levels of testosterone had a 33% increased risk of death over an 18-year period than those with higher levels.
They appeared significantly more likely to have a cluster of risk factors associated with cardiovascular disease and diabetes.
This raises the possibility that men with low testosterone levels may be prone to depression because they are also more likely to be in poor physical health.
However, the Australian researchers concluded that this could not fully explain the link, and that some other factor must also be in play.
Testosterone replacement therapy has also been shown to help elderly men with mild Alzheimer's disease.
Research has suggested that levels of testosterone in men of all ages are falling.
Professor David Kendall, an expert in pharmacology at the University of Nottingham, said there was a wealth of evidence to show that testosterone levels were linked to mood.
For instance, farmers had long castrated their stock to pacify them.
Research on animals had also shown that removal of their gonads blocked the action of anti-depressants on key mood-controlling chemicals in the brain.
"It would be no surprise that low testosterone reduces mood," he said.
"Testosterone therapy offers a relatively simple intervention, potentially, for some groups of older depressives with hypogonadism (low production of sex hormones)."
Professor Stafford Lightman, a hormone expert at the University of Bristol, said testosterone potentially had many small effects which could raise the risk of depression. For instance, low levels had been linked to poor cognitive performance.
However, he warned that depression, particularly in elderly people, was often the result of many different, inter-relating factors, and warned against placing too much emphasis on one in isolation.
"My view is that low testosterone could be a contributory factor to depression, but probably not a very powerful one," he said.