Hormone release from the thyroid

The simple cuboidal epithelium that lines the follicles produces the thyroglobulin for storage. In order to synthesize thyroxin (T4) and triiodothyronine (T3), thyroglobulin (the colloid) must be taken back into the cells by macropinocytosis and the resulting vesicles fused with lysosomes. The proteases in the lysosomes degrade the thyroglobulin, and the released hormones leave the lysosomes and the cell by undefined means. A second cell type called C-cells ("clear" cells for their lesser staining) originates from the ultimobranchial element of the fourth branchial pouch. These are located among the thyroid follicular cells and near the capillaries; they do not touch colloid. They are lightly stained and large, and they secrete calcitonin in response to elevated [Ca 2+] in the blood.