Thyroid gland. Coloured scanning electron micrograph (SEM) of a fracture through the thyroid gland revealing several follicles (green and blue). Between the follicles is connective tissue (pink). A follicle consists of a layer of epithelial cells around a central storage chamber. The cells produce the thyroid hormones tri-iodothyronin (T3) and thyroxine (T4) and secrete them into the central chamber where they are stored as the glycoprotein thyroid colloid. Thyroid hormones regulate the body's metabolism.