What makes you gain weight fast?

The frustration of seeing extra pounds on the scale—or of a too-tight waistband that you know used to fit—is understandable. But did you know that anything from a hormonal imbalance to vitamin deficiencies to the prescription meds you take can hold clues to what's making you gain weight? "A lot of people make what we think are lifestyle choices but are actually our bodies reacting to factors we can't control, " says Robert J. Hedaya, MD, clinical professor of psychiatry at Georgetown University Medical Center. "Whether it's hormonal, a medication side effect, or something else, too often we put the onus on the individual, and there are factors that sometimes justify a doctor's help." (Looking to conquer your weight issues? Prevention has smart answers—get a FREE trial + 12 FREE gifts.)

Here, seven health issues that could be standing between you and your ideal weight—and how to fix them.