Thyroid follicle. Coloured scanning electron micrograph (SEM) of a freeze-fractured thyroid gland showing a colloid follicle (purple). Thyroid follicles store colloid, a solution containing the inactive forms of the thyroid hormones tri- iodothyronine and thyroxine. These hormones help to regulate the metabolism. The follicles are lined by cuboidal epithelium (brown). The follicle at centre fractured between the colloid and the epithelium, revealing an imprint of the epithelial cells on its surface. The surrounding follicles fractured through the colloid, and so appear flat and featureless.