Thyroid disease, or hyperthyroidism, is a condition that affects the thyroid gland. On the front of the neck, this gland is a little butterfly-shaped structure. It produces the hormones tetraiodothyronine (T4) and triiodothyronine (T3), which control how much energy cells need. The thyroid gland, which also controls your metabolism, produces T3 and T4 hormones. Hyperthyroidism occurs when the thyroid generates too much T4, T3, or both. A diagnosis of an overactive thyroid and treatment of the underlying disease can help to reduce symptoms and prevent issues.