CHICAGO - Low levels of testosterone, the male sex hormone associated with virility and energy, may also indicate if a man will die soon, a study said on Monday.

Men in the lowest of three categories of testosterone levels were at least 68 percent more likely to die in the following 4-1/2 years than those in the "normal" category, the study published in the Archives of Internal Medicine said.

Out of a total of 858 military veterans aged 40 or older, 166 had below normal testosterone levels and 35 percent died during the study period. That compared to a 20 percent death rate for subjects with normal testosterone levels.

Levels of testosterone drop when serious illness strikes and recover when the illness passes, or do not rebound if health fails to improve, said study author Molly Shores of the University of Washington at Seattle.

The study said any impact on the study from men who supplemented their testosterone levels was likely minimal.

Cyclist Floyd Landis had a positive doping test for testosterone after a key stage of his Tour de France victory, though the 30-year-old has said the testing was flawed.