The finding might help doctors decide to test for testosterone levels while patients are fasting, the researchers said.

The study, slated for presentation Saturday at The Endocrine Society's annual meeting in Washington, D.C., involved 74 men, including 42 with normal blood sugar, 23 with impaired blood sugar levels ("pre-diabetes") and nine newly diagnosed with type 2 diabetes.

Each of the men drank a sugary solution (75 grams of pure glucose), which typically triggers a spike in blood sugar levels. They then had their testosterone levels tested.

The researchers found that, regardless of whether the men had diabetes or not, blood levels of testosterone dropped by as much as 25 percent after they drank the sugary drink. This trend continued for more than two hours after the glucose was ingested. In fact, 15 percent of the 66 men with normal testosterone levels before the test had low testosterone ("hypogonadism") at some point during the test.

Changes in insulin levels didn't seem to affect the results, nor did levels of other hormones, the team said.

Doctors need accurate testosterone testing to see whether men are hypogonadal and require testosterone supplementation, study co-author Dr. Frances Hayes, an endocrinologist at St. Vincent's University Hospital in Dublin, Ireland, said in a news release from the Endocrine Society. Because testosterone now seems tied to blood-sugar levels, "this research supports the notion that men found to have low testosterone levels should be reevaluated in the fasting state," said Hayes, who did the research while at Massachusetts General Hospital in Boston.

More information

There's more on testosterone at the U.S. National Library of Medicine.