Illustration: Yasmine Gateau

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Your thyroid, the butterfly-shaped gland in your neck, has a mighty job: producing hormones that regulate nearly every system in the body, including metabolism, energy and brain function. But it's also prone to go on the fritz. More than 12 percent of Americans will develop a thyroid disorder in their lifetime, but up to 60 percent of them won't know it because the conditions are difficult to diagnose. The classic symptoms— sluggishness, weight loss, weight gain—can mimic those of chronic stress or other conditions and, as a result, can be overlooked by sufferers and their doctors. Don't let that happen to you.