Your thyroid is a butterfly-shaped gland in your neck, just above your collarbone. It is one of your endocrine glands, which make hormones. The thyroid helps set your metabolism - how your body gets energy from the foods you eat.

Millions of people in the U.S. have thyroid diseases. Most of them are women. If you have a thyroid disease, your body uses energy more slowly or quickly than it should. A thyroid gland that is not active enough, called hypothyroidism, is far more common. It can make you gain weight, feel fatigued and have difficulty dealing with cold temperatures. If your thyroid is too active, it makes more thyroid hormones than your body needs. That condition is hyperthyroidism. Too much thyroid hormone can make you lose weight, speed up your heart rate and make you very sensitive to heat.

There are many causes for both conditions. Treatment involves trying to reset your body's metabolism to a normal rate.