Thyroglobulin, produced by the follicular cells of thyroid gland is used to produce the thyroid hormones via a number of steps. Thyroglobulin contains some 100-120 tyrosine residues, of which only a small fraction is used to produce thyroid hormones. Usually there are 5 to 6 molecules of either thyroxine (T4) and triiodothyronine (T3) derived from each molecule of thyroglobulin.