Light micrograph of a thyroid gland showing the follicles. The follicles are lined by a single layer of cuboidal epithelial cells (dark purple, dark nuclei), which synthesize 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 (purple), which is a storage form of the thyroid hormones. When actively secreting hormones the lining cells are large and cuboidal, when less active the follicles are distended and the secretory cells flattened.