thyroglobulin

Thyroglobulin is best pictured as a workbench and a storage shelf rolled into one giant protein. It is the scaffold on which thyroid hormones are built and the form in which they are stockpiled inside the colloid until the body calls for them.

Follicular cells make this large protein and secrete it into the colloid. There, thyroid peroxidase attaches iodine atoms to specific tyrosine building blocks within thyroglobulin and then couples those iodinated pieces together to form thyroxine and triiodothyronine while they are still part of the protein chain. When hormone is needed, the cell engulfs a droplet of colloid and enzymes chop the thyroglobulin apart, freeing the finished hormones for release.

In medicine, thyroglobulin doubles as a tumor marker. After surgery and radioiodine treatment for differentiated thyroid cancer, blood thyroglobulin should fall to very low levels; a later rise suggests that thyroid tissue — possibly cancer — has returned. This use only works when thyroid tissue is meant to be absent, so it is unreliable in people who still have a normal thyroid.