The thyroid gland is a butterfly-shaped organ that is a part of the body's endocrine system, the system that controls hormone production and distribution. It's a small gland that sits in front of the neck and regulates the hormones T3 and T4, which control weight; energy levels; body temperature; and skin, hair, and nail growth, among other things. They also help regulate your heartbeat, breathing and metabolism (the speed at which you gain or lose weight).