The hormone that regulates metabolism is called thyroid hormone, specifically Thyroxine (T4) and Triiodothyronine (T3), produced by the thyroid gland. These hormones influence how the body uses energy, affects growth, and affects the metabolic rate. An imbalance in thyroid hormones can lead to various health issues, such as hyperthyroidism or hypothyroidism, which can dramatically alter energy levels and weight management. Regular health check-ups can help monitor thyroid function.