Thyroid hormone refers to a group of hormones produced by the thyroid gland, which is located in the neck. These hormones play an important role in regulating metabolism and growth, as well as controlling other body functions such as heart rate and blood pressure. The two main types of thyroid hormones are triiodothyronine (T3) and thyroxine (T4). They help to maintain the body's energy balance, stimulate protein synthesis, and regulate the way cells use energy. An imbalance in thyroid hormone levels can lead to various health issues such as hypothyroidism or hyperthyroidism.