Male infertility could be a warning sign for certain types of cancer, a new study says.

Men with severely reduced fertility are more likely to develop colon cancer or thyroid cancer, researchers reported in the European Journal of Epidemiology.

“They were found to have almost twice the risk of developing colorectal cancer and three times the risk of thyroid cancer compared with men who become fathers naturally,” researcher Michael Kitlinski said in a news release. He’s a medical student at Lund University in Sweden. 

The findings add to prior studies that have linked lower male fertility to a wide variety of health problems, researchers said.

Men with no sperm in their semen are among those at most risk for serious illnesses, while men with good sperm quality tend to live longer on average, researchers said.

“Reproductive capacity is linked to an individual’s genetic makeup,” senior researcher Angel Elenkov, a docent in reproductive medicine at Lund University, said in a news release. "One theory is that if something goes wrong at the genetic level – which can manifest as reduced sperm quality – other systems in the body may also be affected, increasing the risk of disease."

For the new study, researchers analyzed data for more than 1.1 million Swedish men who had their first child between 1994 and 2014.

The team compared the health of more than 14,500 men who became fathers with the help of assisted reproduction against those who achieved pregnancy naturally.

Researchers noted that although the relative cancer risk was higher for men with poor sperm quality, cancer remained rare overall among young fathers.

“Most men who undergo fertility testing are between 30 and 35 years old,” Elenkov said. “The aim is to help them become fathers, and there is no subsequent follow-up on their health.”

The findings are significant, he said.

“These findings are important from a public health perspective given that cancers of the colon and rectum, as well as thyroid cancer, are on the rise among younger people," Elenkov said. "These cancers could be prevented through early screening.”

The team emphasized that fertility treatments do not contribute to cancer, but are merely a sign of other health problems.

Researchers plan to delve deeper into these men’s cancer risk, looking for the exact factors that might make them susceptible to specific types of cancer.