thyroglobulin
Meaning of thyroglobulin: The scaffold protein on which thyroid hormone is assembled and stored, and the blood marker used to watch for returning thyroid cancer.
Definition of thyroglobulin
Thyroglobulin is a large protein made only by thyroid cells and stored in the jelly-like material at the center of each thyroid follicle. Thyroid hormone is built on it: iodine is attached to particular building blocks along the protein chain, and finished hormone is later cut free and released into the blood. Small amounts of the protein itself leak into the bloodstream too.
How is thyroid hormone built on it?
Follicular cells secrete the protein inward, into the sealed space at the center of the follicle. Iodide taken up from the blood is attached to tyrosine units along the chain, and two iodinated units are then joined while both are still part of the protein. That joining creates the hormone molecule. When the body needs thyroid hormone, the cell takes a droplet of the stored material back inside and enzymes cut the hormone loose. The arrangement lets the gland hold weeks of hormone in reserve.
Why is it measured?
Because only thyroid tissue makes it. After the gland has been removed for differentiated thyroid cancer, and any remnant destroyed with radioactive iodine, the level in blood should be very low or undetectable. A rise afterward suggests thyroid tissue has reappeared, which is why it is followed at intervals as a marker of recurrence. It is of no use for diagnosing cancer in a gland that is still present, since inflammation, an overactive gland and simple goiter all raise it.
What interferes with the test?
| Interference | Effect on the result |
|---|---|
| Antibodies against thyroglobulin | Falsely low on the usual assay, so they are measured alongside it |
| A different laboratory method | Results are not interchangeable, so trends are followed on one assay |
| The level of thyroid stimulating hormone | Stimulation raises thyroglobulin, so the conditions of the test are recorded |
| Other interfering antibodies | Occasionally give a falsely high reading |
What is it not?
It is not a thyroid hormone, and it is not thyroxine-binding globulin, the carrier protein that ferries hormone through the blood. The similar names cause frequent confusion. It is also not a screening test: measuring it in someone who has never had thyroid cancer gives a number with no useful meaning.
Does the body react against it?
Sometimes. Because the protein is normally sealed inside the follicle, the immune system has limited exposure to it, and in autoimmune thyroid disease antibodies against it appear in the blood. Those antibodies are themselves a marker of thyroid inflammation, and they are the reason the tumor marker result has to be interpreted with an antibody level beside it.
Also known as
Used in a sentence
Thyroglobulin remained undetectable with negative antibodies at follow-up after total thyroidectomy.
Good to know
Thyroglobulin is useful as a tumor marker only once the thyroid has been removed, and antibodies against it, present in a substantial minority of people, push the measured level falsely low.
Word origin
Where “thyroglobulin” comes from
Thyroid comes from the Greek thyreos, an oblong shield, with eidos, meaning form; globulin comes from the Latin globulus, a little ball, used for a family of proteins.