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Britannica Sources used in this answer

How are thyroxine and triiodothyronine made in the thyroid gland?

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A labeled diagram highlights the periodic table entry for iodine, atomic number 53, symbol I, atomic weight 126.90447, and electron configuration. It shows iodine as a solid halogen with an orthorhombic crystal structure and strongly acidic higher-valence oxides, using color and shape keys for classification.
iodine

The synthesis of thyroxine and triiodothyronine in the thyroid gland is a complex process dependent on the availability of iodine and the amino acid tyrosine.12

The process occurs through the following steps:

  • Iodide Uptake: Thyroid follicular cells actively concentrate inorganic iodide from the bloodstream.12
  • Oxidation and Binding: The iodide is oxidized by the enzyme iodide peroxidase and attached to tyrosine residues within thyroglobulin molecules, forming iodotyrosines (monoiodotyrosine and diiodotyrosine).12
  • Hormone Formation: These iodinated tyrosine residues are rearranged and joined to form the active hormones: thyroxine (containing four iodine atoms) and triiodothyronine (containing three iodine atoms).12
  • Storage and Release: Thyroglobulin serves as the structural scaffold for this synthesis and as a storage form within the thyroid colloid. When needed, the thyroid cells take up droplets of this colloid, and enzymes hydrolyze the thyroglobulin to release the hormones into the bloodstream.12