A player is carted off of the field after sustaining a concussion in an NFL game.

Head trauma in former NFL players may be linked to low testosterone and erectile dysfunction later in life, according to a study by Harvard researchers.

More than 3,400 former professional football players, the largest group studied to date, were surveyed by investigators at the Harvard T.H. Chan School of Public Health and Harvard Medical School as part of the university's ongoing Football Players Health Study. They found that former NFL players who previously reported concussion symptoms after head injuries were more likely to report erectile dysfunction and low testosterone levels.

"We found the same association of concussions with ED among both younger and older men in the study, and we found the same risk of ED among men who had last played 20 years ago," said Andrea Roberts, one of the study's senior authors. "These findings suggest that increased risk of ED following head injury may occur at relatively young ages and may linger for decades thereafter."

The study was published Monday in JAMA Neurology.

Of all the study's participants, 18% reported low testosterone, almost 23% reported ED and slightly less than 10% of participants reported both symptoms.

Players who reported the most concussion symptoms had nearly twice the risk of ED as those reporting the fewest symptoms. Notably, even former players with relatively few concussion symptoms had an elevated risk for low testosterone, which researchers think suggests that there may be no safe threshold for head trauma.

Those findings didn't change based on the player's position on the field either.

"We threw all those things into the model and in that scenario position never lit up. There was never enough noise — we had 10 position category's and just didn't see any effect," said Rachel Grashow, the study's lead author.

That ED risk persisted even when researchers accounted for other possible causes for the condition.

"We were able to control for so many other reasons why the guys would have ED, hypertension, obesity, cholesterol, depression, anxiety," said Grashow. "During all of these other conditions that football players are especially susceptible to, like sleep apnea, you still see this incredibly strong relationship," said Grashow.

One possible explanation for the results could be that injury to the brain's pituitary gland causes hormonal changes like diminished testosterone and ED.

That explanation is echoed in previous studies that saw higher ED risks and neurohormonal dysfunction among people with head trauma and traumatic brain injury, including military veterans and civilians with head injuries.

The researchers caution that their findings are observational — based on self-reported concussion symptoms and indirect measures of ED and low testosterone.

"This is definitely a gateway study. We definitely do not feel we've said anything affirmative about that relationship," said Grashow.

She added that in the context of the other studies that have been done, this is still an important finding.

"I think it's strong evidence that there is a real biologically probable link between head injury during play and erectile dysfunction and low testosterone," said Grashow.

The findings also suggest that sleep apnea and use of prescription pain medication contribute to low testosterone and ED, though it remains unclear whether that relationship is a consequence of head injury or another independent finding, the researchers said.

Grashow hopes the findings will help to destigmatize the issue for NFL players and push men to get help.

"We want to say to them, look this isn't a personal failure or a failure of your masculinity; it might actually be tied to a very specific biological mechanism that's treatable," she said.

The research was supported by the National Football League Players Association. The Football Players Health Study examines various aspects of players' health over the course of their lives.