If you're an HIV-positive guy on the lookout for low testosterone—and you should be; hypogonadism, or low T, is common in men with the virus and can cause problems like fatigue, depression and bone loss—make sure you and your doc are checking for both “total” and “free” levels of the hormone in blood samples that are collected first thing in the morning.

“Using total testosterone to diagnose hypogonadism will result in missing about 30 percent of the cases,” says Anne Monroe, MD, MPH, an author of a recent study at Johns Hopkins University. She says that free testosterone, which is the proportion of total testosterone not bound to a protein (SHBG) that can prevent the hormone from working as it should, is the test to keep an eye on.

“Any man who has symptoms of hypogonadism with a low free T”—even if the total T level is within the normal range—“should be considered for testosterone therapy,” she says, adding that “older and obese men are at higher risk for low testosterone. Increasing physical activity to lose weight is a great way to increase testosterone without hormone replacement.”