Thyroid hormone synthesis requires the ingestion of iodine (about 1mg per week). Sources include iodized table salt, seafood, seaweed, and vegetables.
After absorption from the gastrointestinal tract, it is removed as required from the blood and moved into the thyroid follicular cell colloid. Here, the iodine is oxidised and combined with thyroglobulin in a process called organification.
After several more combinations with aminoacids T3 and T4 are formed ready for secretion.