Your thyroid is in charge of releasing thyroid hormones (duh), and controlling your metabolism. If your thyroid gland is under-producing, that could lead to weight gain and a total lack of energy, among other symptoms. "Hypothyroidism, or low thyroid function, is a common and under-diagnosed cause of fatigue," says Dr. Jennifer Haythe, cardiologist and assistant professor of medicine at Columbia University Medical Center. "Constipation, dry skin, weight gain, thinning hair, and depression are just some of the other side effects of an underactive thyroid."
If you're experiencing these symptoms, check with your doctor, who will perform a simple blood test to be sure. Treatment usually includes medication.
Diabetics don't process glucose properly, which is a major problem since glucose is the body's main source of fuel. "As a result, circulating blood sugar levels are high, resulting in excessive urination, dehydration, blurry vision, and thirst. This leads to fatigue and exhaustion," Dr. Haythe says. Diabetes affects more than 29 million Americans -- 8 million of whom go undiagnosed.