FALLACY: Many people who struggle with their weight are likely Suffering from a thyroid disorder

FACT: Thyroid disorders are fairly rare among the general population

Just like everything else in our bodies, hormones have an impact on metabolism, too. Thyroid hormones are the principle ones when it comes to influencing your metabolic rate. "People who have high thyroid function have very high metabolic rates, and those who have hypothyroid or underfunctioning thyroids have slow metabolic rates," Keith remarks. But before you can pinpoint your weight problem on hypothyroidism, you should know that it isn't a common ailment. Only about 4% of Americans have some sort of thyroid disorder or disease. The only way to be diagnosed is by your physician, who will probably test you only if you elicit symptoms.

There are other hormones, such as adrenaline (aka epinephrine), that are involved in determin- ing your caloric expenditure. In the case of adrenaline, something as simple as drinking a cup of coffee can cause this hormone to boost your metabolism. "[Caffeine] increases your alertness by increasing adrenaline levels at least to par. It stimulates; the heart rate goes up some," says Keith. "That's the major effect of caffeine for calorie burn and metabolism -- its effect on hormones." (He also points out that caffeine can be used to burn some calories and help control weight when used wisely and in moderation.)

FALLACY: When you hit a sticking point during weight loss, it's due to a stalled metabolism

FACT: Plateaus are a sign that your metabolism is doing its job

When trying to lose weight, most of us have hit the dreaded sticking point -- when the scale won't budge, your clothes aren't getting looser and frustrations begin to rise. It's the perfect time to re-evaluate your eating and exercise plan, and consider: How much weight have you lost so far?

"As most people find out, they can lose 10% of their bodyweight before it becomes more difficult to lose another 10%," Keith says. "At about that point, the weight won't come off as fast as it once did."

There are two reasons why: First, you're smaller, so your metabolism is matching the "new" you; second, your body thinks it's starving, and as a result, thyroid hormone levels drop and your resting metabolic rate may drop 10%—25%, so you aren't burning as many calories in a day as you were. And here's what most of us forget: When we lose weight, we become lighter, meaning the amount of calories we burn in everyday activities drops as well. Eventually your metabolism adjusts to your new, smaller body.