Figure 1: Thyroglobulin and thyroid peroxidase in thyroid hormone biosynthesis. The essentiality of thyroglobulin and thyroid peroxidase is shown in this simplified diagram of thyroid hormone biosynthesis. Thyroglobulin acts as the skeleton in thyroid hormone synthesis. It is synthesized and glycosylated in the rough endoplasmic reticulum, and is exocytosed through the apical membrane. Thyroid peroxidase acts in three parts (a) it oxidizes iodine, (b) organifies iodide to the tyrosine residues to the thyroglobulin forming monoiodotyrosine and di-iodotyrosine (c) Couples monoiodotyrosine and di-iodotyrosine to either T4 or T3 (TPO: thyroid peroxidase, Tg: thyroglobulin, MIT: monoiodotyrosine, DIT: di-iodotyrosine, T4: thyroxine, and T3: triiodothyronine).