Chapter 17 The endocrine system/Fig 17.08.jpg

You can download this figure by right-clicking it and choosing 'Save Picture As...'

The synthetic pathway and secretion of thyroid hormones.Inorganic iodide is trapped (1) and is oxidized within the follicular cells by the action of thyroid peroxidase (2). It is then secreted into the colloid and undergoes organification close to the cell membrane by the iodination of tyrosyl residues on thyroglobulin (Tg), also secreted by the follicular cells (3). First, mono-iodotyrosine (MIT) and di-iodotyrosine (DIT) are produced. Thyroxine (T₄) is then formed by the coupling of two molecules of DIT, and tri-iodothyronine (T₃) by coupling of MIT and DIT. When hormone is required, thyroglobulin is reabsorbed (4) and undergoes proteolysis (5), releasing T₃ and T₄ (6) which diffuse into the circulation (7). The excess, along with MIT and DIT, undergoes deiodination (8).