Iodide trapping is not catalysed by thyroid peroxidase. Oxidation, organification and coupling are catalysed by it. Steps in synthesis and secretion of thyroid hormone: Thyroglobulin synthesis by thyroid follicular cells Iodide trapping:Na+ - I- sympoer (NIS) actively transpos I- into the follicular cells. This is inhibited by thiocyanate and perchlorate anions and is stimulated by TSH. Iodide is transpoed into the lumen of the follicle by I -- Cl- transpoer called Pendrin. Oxidation of I- to I2 catalysed by the thyroid peroxidase (TPO). Organoification/ Iodination of tyrosine residues of thyroglobuli: Binding of I2 to tyrosine residues of thyroglobulin to form MIT (monoiodotyrosine) and DIT (Diiodotyrosine). Catalysed by TPO. Coupling reaction mediated by TPO: MIT+DIT - T3, DIT - T4. Storage and release: T3 and T4 are stored bound to thyroglobulin. When stimulated, they are cleaved from thyroglobulin and released into the circulation. Ref: Guyton and Hall 13th edition Pgno: 951