Are you frustrated by diets that don't work? Have you put on pounds that can't be explained by trips to the drive-through? You may have a thyroid problem. "The thyroid gland is famous for affecting our weight," says Scott Isaacs, M.D., author of Hormonal Balance: Understanding Hormones, Weight, and Your Metabolism.

More than 10 million Americans have been diagnosed with thyroid troubles, but an additional 13 million may be unaware that they have the condition, according to a University of Colorado Health Sciences Center study.Here’s how to tell if you should schedule a thyroid checkup with your doctor.

The Front Burner

"Think of your thyroid as the thermostat of the body," says Richard Shames, M.D., co-author of Feeling Fat, Fuzzy, or Frazzled? Located in the lower part of your neck, the gland regulates your metabolism, which means it influences how easily you burn calories, how warm you feel and whether or not you gain weight. When your thyroid gland stops churning out hormones properly, a surprising number of internal processes go out of whack, including your digestion and ability to concentrate.

What Goes Wrong

These are the three most likely thyroid problems:

1. Hypothyroidism: An underactive thyroid, which means that too little of the appropriate hormones are circulating throughout your body. You may feel tired, gain weight and experience constipation.
2. Hyperthyroidism: An overactive thyroid indicates you have an oversupply of the gland’s hormones, which can lead to anxiety, insomnia, diarrhea and weight loss. In the most common thyroid trouble — Graves’ disease — the excess hormones also lead to abnormal thickening of the skin on the lower legs, plus eye problems. Some experts think that fluctuating levels of estrogen in the body can trigger Graves’ disease.
3. A Growth: Having a lump on this gland is not uncommon and, fortunately, 95 percent of lumps are benign, says Mary J. Shomon, author of The Thyroid Hormone Breakthrough. Still, since a lump could impair how much hormone is secreted from the gland, get any unusual growth examined by a doctor. For a do-it-yourself checkup, see the next page of this article.

Tip-Offs to Trouble

Weight gain isn't the only symptom of thyroid issues. Answering "yes" to one or more of these questions is another sign that something’s off-kilter.

• Your hands: Are they retaining water? Are your fingernails brittle, and do they break easily?
• Your eyes: Are they dry or sensitive to light? Are your eyelids puffy? Are the outer thirds of your eyebrows thinning?
• Your neck: Does it feel heavy or sensitive to the touch?
• Your voice: Does it get hoarse?
• Your temperature: Are you cold in the summer or warm in the winter?

Next page: Who's most at risk