Thyroid gland follicles. Coloured transmission electron micrograph (TEM) of follicles from the thyroid gland. A follicle consists of a layer of epithelial cells around a central storage chamber. The cells produce the thyroid hormones tri-iodothyronine (T3) and thyroxine (T4) and secrete them into the central chamber where they are store as the glycoprotein thyroid colloid. Two parafollicular cells are also present (purple). Parafollicular cells secrete the hormone calcitonin. The follicle is surrounded by blood vessels which transport the hormones around the body. Thyroid hormones regulate the body's metabolism. Magnification: x800 when printed at 10 centimetres across.