Up to 20 million Americans have a thyroid condition — and up to 60 percent of them don’t know it. The thyroid can be either overactive or under-active. And both conditions can lead to big health problems. The thyroid is found in the middle of the lower neck. The hormones it produces (T3 and T4) affect every cell in the body. They help control your body temperature and heart rate; and help regulate the production of protein. If the thyroid produces too much — or too little T3 and T4, it can create problems. Specifically, those conditions are hyperthyroidism and hypothyroidism.