The hormone primarily responsible for regulating metabolism is thyroxine, produced by the thyroid gland. Thyroxine (T4) helps control the pace of metabolic processes, including how quickly the body uses energy and converts food into fuel. By influencing metabolic rate, thyroxine plays a crucial role in body weight regulation and energy balance. Thyroid disorders can lead to weight gain or loss, highlighting the importance of a well-functioning thyroid.