Illustration of a single human thyroid follicle, the functional unit of the thyroid gland. The follicle has been cut open to reveal the internal layer of epithelial cells surrounding a pool of thyroid colloid (purple). The blood supply is also featured (red & blue). The follicular cells secrete two forms of thryoid hormone, thyroxine (T4) and triiodothyronine (T3), which are stored in the jelly-like colloid. When actively secreting hormones the lining cells are large & cuboidal, & the colloid is diminished. When less active the colloid is distended & the secretory cells flattened.