How does the thyroid gland make thyroxine and triiodothyronine?

The thyroid gland synthesizes thyroxine (T4) and triiodothyronine (T3) through a process centered in the thyroid follicular cells, which utilize iodine and the amino acid tyrosine acquired from the diet.1

The synthesis involves several key steps:

• Concentration and Oxidation: Follicular cells concentrate iodine from the serum. This iodine is then oxidized and attached to tyrosine residues within thyroglobulin molecules, forming compounds known as iodotyrosines.1
• Rearrangement: These iodinated tyrosine residues are rearranged to form thyroxine (containing four iodine atoms) and triiodothyronine (containing three iodine atoms). Thyroglobulin serves as the structural scaffold for this synthesis and as a storage form for the hormones within the colloid of the thyroid follicles.12
• Release: When hormones are required, thyroglobulin is reabsorbed from the colloid into the follicular cells, where enzymes split it into its component parts, releasing T4 and T3 into the circulation.2

While the thyroid produces both, it secretes significantly more thyroxine, which often acts as a circulating precursor that is converted to the more active triiodothyronine in peripheral tissues.1