Thyroid: The gland in the throat that synthesizes thyroid hormones that affect metabolism. The thyroid is a butterfly-shaped gland that sits on the lower portion in front of the neck. The thyroid gland secretes several types of thyroid hormones that work together to control important body functions. The main thyroid hormone is thyroxine , also called T4. Thyroid hormones act as chemical messengers throughout the body influencing metabolism, energy, weight, water retention, growth, development, and body temperature.