Your thyroid is a butterfly-shaped gland located below your Adam's apple. While you probably never think twice about what it does for you, when it starts to go wonky you'll wonder why no matter what your efforts are you just can't lose weight.
The way the thyroid works is that it releases hormones that control many bodily functions, including, you guessed it, your metabolic rate. When your thyroid gland slows down, even on the slow side of normal, your metabolic rate also slows down. Conversely, if it's overactive, your metabolic rate speeds up. Your primary doctor can test your thyroid function, and if needed prescribe synthetic thyroid hormones to help your lagging gland.