Thyroid gland. Light micrograph of a section through a healthy thyroid gland showing the follicles (large cells). The follicles are lined by a single layer of cuboidal epithelial cells (purple, dark nuclei), which synthesise and secrete the iodine-containing hormones T3 (tri-iodothyronine) and T4 (thyroxine). The thyroid follicle is filled with a glycoprotein substance called thyroglobulin, or thyroid colloid (pink), which is a storage form of the thyroid hormones.