How do thyroid hormones enter the bloodstream from follicular cells?

When thyroid hormones are required by the body, they are released from the thyroid gland through a multi-step process involving the follicular cells and the stored colloid within the follicles.1

The process occurs as follows:

• Reabsorption: The follicular cells take up droplets of the stored thyroid colloid, which contains the prohormone thyroglobulin, from the follicular lumen.12
• Hydrolysis: Once inside the follicular cells, the thyroglobulin is hydrolyzed (broken down) by enzymes into its component parts, which include the active thyroid hormones thyroxine (T4) and triiodothyronine (T3).12
• Release: These hormones then pass out of the follicular cells and enter the circulation to be transported throughout the body.12

While iodotyrosines are also produced during the breakdown of thyroglobulin, they are typically not released into the bloodstream; instead, an enzyme removes the iodine from them so it can be conserved and reused for further hormone synthesis.2