Conversion of iodide ions to an oxidized form of iodine, either nascent iodine (I0) or I3-, which is then capable of combining directly with the amino acid tyrosine. This oxidation of iodine is promoted by the enzyme peroxidase and its accompanying hydrogen peroxide, which provide a potent system capable of oxidizing iodides. Iodide exit from the thyrocyte across the apical membrane to access the colloid is mediated by a Cl-/I- exchanger known as pendrin. The mechanism of action of propylthiouracil is paly to block the peroxidase enzyme that is required for iodination of tyrosine and paly to block the coupling of two iodinated tyrosines to form thyroxine or triiodothyronine. Reference: Guyton; 13th edition; Page no: 953, 960Reference: Ganong's Review of Medical Physiology; 25th edition; Page no: 339