Thyroid Hormone Synthesis – Iodine Oxidation This ‘trapped’ iodide is oxidised by the thyroid peroxidase enzyme (TPO) and linked to tyrosine molecules in thyroglobulin to form monoiodotyrosines (MIT) and diiodotyrosines (DIT), neither of which are metabolically active. The iodotyrosines are formed within the thyroglobulin molecules (which are synthesised by the follicular cells and secreted into the lumen).