The thyroid follicle (or thyroid vesicle) is the morpho-functional unit of the thyroid gland. The follicle is composed of an uni-stratified epithelium of follicular cells, which is producing thyroid hormones, disposed around a central lumen containing the colloid. The colloid is mainly composed of the precursor of thyroid hormones, the thyroglobulin. The thyroid follicle is a real trap of iodine, essential element to the functioning of the body; the ion iodide will be captured and stored in the colloid. The biosynthesis of thyroid hormones thyroxine (T4 ) and triiodothyronine (T3 ) can then take place from iodine coupling to thyroglobulin.
The thyroid follicle, outside a majority of follicular cells, contains 1 to 2 % of said parafollicular cells (or C cells) producing calcitonin. They do, however, have not contact with the colloid.