How do follicular cells release thyroid hormones from thyroglobulin?

When thyroid hormones are required, follicular cells initiate the release process by reabsorbing thyroglobulin from the colloid stored in the follicular lumen.12

Once inside the follicular cells, the thyroglobulin is hydrolyzed—broken down through a reaction involving water—by specific enzymes. This process splits the thyroglobulin into its component parts, including the thyroid hormones thyroxine (T${}_4$) and triiodothyronine (T${}_3$). Following this breakdown, the hormones are released from the cells into the circulation.12

During this process, iodotyrosines are also produced; however, these are not hormonally active. An enzyme called deiodinase removes the iodine from these iodotyrosines, allowing the iodine to be conserved and reused for future hormone synthesis.2