Why is iodine needed to make thyroid hormones?

Iodine is essential for the production of thyroid hormones because it is a fundamental structural component of both thyroxine (T4) and triiodothyronine (T3). These hormones cannot be synthesized in the absence of iodine.1

Biochemically, the synthesis process involves the following steps:

• Incorporation: Thyroid follicular cells concentrate iodine from the serum and oxidize it.1
• Binding: The iodine is attached to tyrosine residues within thyroglobulin molecules to form iodotyrosines.1
• Synthesis: These iodinated tyrosine residues are rearranged to create the final hormone structures. Thyroxine contains four iodine atoms, while triiodothyronine contains three.1

Because these are the only biologically active substances in the body that contain iodine, an adequate dietary intake is critical for maintaining normal thyroid gland function, which regulates metabolism, growth, and other vital physiological processes.12