From dietary iodide to T4 and T3
The thyroid gland builds its hormones around iodine. Circulating iodide is actively pulled into thyroid cells by the sodium-iodide symporter (NIS), then oxidised and attached to tyrosine residues on the large protein thyroglobulin by the enzyme thyroid peroxidase (TPO). Coupling these iodinated tyrosines produces thyroxine (T4, four iodines) and the more active triiodothyronine (T3, three iodines), which are later released into the blood.
The diagram follows this assembly line, NIS to TPO to T4 and T3, showing that iodine is literally a building block of the hormone, not just a regulator.
Why both too little and too much iodine cause problems
Iodine deficiency limits hormone production and is the leading preventable cause of goitre and developmental impairment worldwide, which is why many countries iodise salt. Crucially, excess iodine is also harmful: it can trigger or worsen thyroid dysfunction in susceptible people, so more is not better. The therapeutic window is narrower than many supplement labels imply.
High-dose iodine supplements can be risky, especially with existing thyroid disease. This is general educational information, not medical advice; thyroid concerns and iodine dosing should be handled by a qualified professional.