Hyperthyroidism is a condition in which the thyroid gland makes too much thyroid hormone. The condition is often referred to as an “overactive thyroid.” Thyrotoxicosis is a toxic condition that is caused by an excess of thyroid hormones from any cause.

Your thyroid is a gland in the front of your neck. It controls your metabolism, which is how your body turns food into energy. It also affects your heart, muscles, bones, and cholesterol. Having too much thyroid hormone can make a lot of things in your body speed up. You may lose weight quickly, have a fast heartbeat, sweat a lot, or feel nervous and moody. Or you may have no symptoms at all.