Stock Photo - Situation, thyroid follicles in section, follicular cell and functioning. In the background, cut thyroid with thyroid follicles. In cartridge, follicular cells in contact with a blood capillary (bottom). Above, a silhouette locating the thyroid gland pituitary gland and hypothalamus. The synthesis of thyroid hormones T3 and T4, which is carried out in follicular cells and the colloid substance of the thyroid follicle, is stimulated by TRH at the level of the hypothalamus, which induces the synthesis of TSH in the pituitary gland. The thyroperoxidase allows the synthesis of iodides provided by the food iodine under the influence of TSH. The MID and DIT precursors are formed which, by coupling, give T3 and T4 previously stored in the thyroglobulin at the level of the colloid substance. The thyroglobulin re-enters the follicular cell to release T3 and T4 into the bloodstream.