Millions of people, most of them women, suffer from thyroid disease. When your body produces too little thyroid hormone, the hormone responsible for metabolism, heart rate, and mood, you are said to have hypothyroidism, or an underactive thyroid. If your body makes too much of the hormone, you're said to have hyperthyroidism, or an overactive thyroid. Thyroid hormone is responsible for everything from your basal metabolic rate—the rate at which your body uses oxygen and energy to function—to the growth of your hair, skin, and nails. But when you don't have the right amount, you may notice changes in bodily functions.