What is the difference between hypothyroidism and hyperthyroidism?

The main difference is that hypothyroidism means your thyroid gland is underactive, while hyperthyroidism means it is overactive. Your thyroid makes hormones that help control your body’s energy use and many other functions. With hypothyroidism, you may feel unusually tired, gain weight, feel colder than usual, have dry skin, or become constipated. With hyperthyroidism, you may lose weight without trying, feel hot or sweaty, have a fast or irregular heartbeat, feel nervous or shaky, or have more frequent bowel movements. These symptoms can overlap with other health problems, so blood tests are usually needed to confirm a thyroid condition. Both can be treated, but the treatment depends on the underlying cause.