Thyroid gland. Coloured light micrograph (LM) of a human thyroid gland showing the follicles. The follicles are lined by a single layer of cuboidal epithelial cells (small grey nuclei), which synthesize and secrete the iodine-containing hormones T3 (tri-iodothyronine) and T4 (thyroxine). Each thyroid follicle contains a glycoprotein substance thyroid colloid (white), 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 more flattened (as here). Magnification: x25 at 35mm size.