If you're over 30, pregnant or have a family history of thyroid problems, take extra care to examine your neck annually: More than half of thyroid conditions (some of which cause "unexplained" weight gain) are undiagnosed, says Alan L. Rubin, M.D., author of Thyroid for Dummies (Hungry Minds, 2001). Here's how to do a neck check: Using a hand-held mirror, look right below your Adam's apple to find your thyroid. (It's a butterfly-shaped gland that wraps around the front of the windpipe.) Tip your head back slightly and take a drink of water. While swallowing, check for any bulges, widening or other enlargement around the thyroid. Repeat. If you notice anything suspicious, call your doctor. A visual aid can help: Go to the Web site of the American Association of Clinical Endocrinologists, www.aace.com, for a downloadable illustration.