Iodine is needed for the synthesis of thyroid hormone. Our main source of iodine is from salt which has been enriched with iodine and which is an ingredient in our bread and other foods. The amino acid tyrosine is also required. Iodide, the form of iodine present in the blood, is transported from the blood via the thyroid gland cell, or follicular cell, into the thyroid follicle (the cavity), where it is converted into the iodine atom.
The follicular cell also makes thyroglobulin and transports it to the thyroid follicle. Thyroglobulin contains tyrosines. With the help of the enzyme TPO, each tyrosine molecule can bind to two iodine atoms. If tyrosine binds with one iodine atom, then monoiodotyrosine or MIT is formed. If tyrosine binds with two iodine atoms, then diiodotyrosine or DIT is produced.
T4 has four iodine atoms and is produced when two DITs (2+2 iodine atoms) bind together. T3 has three iodine atoms and is produced when one DIT and one MIT (2+1 iodine atoms) bind together.