idase (TPO) and incorporation into tyrosyl residues along the thyroglobulin (Tg) backbone. The thyroid hormones T3 and T4 are synthesized by coupling of two iodotyrosine residues and stored in the colloid. All of these steps are stimulated by pituitary-derived thyroid stimulating hormone (TSH), which interacts with the TSH receptor at the basolateral membrane of thyroidal cells, through the cAMP pathway (Figure 1).