Thyroid gland. Coloured scanning electron micrograph (SEM) of a fracture through the thyroid gland revealing several follicles (purple and blue). Between the follicles is connective tissue (brown). A follicle consists of a layer of epithelial cells (blue) 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 (purple). Thyroid hormones regulate the body's metabolism.