Thyroglobulin, a protein synthesized by the thyroid gland, is the precursor of two thyroid hormones, well known as T4 and T3. The thyroglobulin assay is mainly used for monitoring patients who have undergone thyroidectomy (removal of the thyroid) following cancer of this gland. Thus, the analysis of the thyroglobulin will make it possible to verify that all the thyroid tissue was removed during the surgery. It is important, after remission, to carry out an annual check-up by blood test in order to ensure the absence of recurrence.

Thyroglobulin is a protein made by the thyroid gland. Thus, thyroglobulin is the precursor of two thyroid hormones: thyroxine (T4) and triiodothyronine (T3), two hormones which influence many functions of the human body. Thyroglobulin is a very abundant protein, but it is only present in thyroid tissue. This is why the presence of even traces of thyroglobulin in the blood means that normal or abnormal thyroid tissue is present.

The thyroid, a small butterfly-shaped organ located in the neck, has a role in particular in controlling the rate at which our body consumes energy. Located in the throat, right up against the trachea, it is made up of follicles (small rounded formations within the tissue that delimit the cavities).

No other part of the body makes thyroglobulin. Thyroid cancers produce them, not only those that are confined to the thyroid gland, but also those in which there has been a swarming of tumor cells throughout the body. The thyroglobulin assay is therefore mainly used as a tumor marker, with the aim of verifying the effectiveness of the treatment of thyroid cancer, or else to monitor the appearance of recurrence.

The analysis of thyroglobulin levels is mainly useful in the monitoring of differentiated thyroid cancer. Indeed, this analysis will thus make it possible to verify that all of the thyroid tissue has been removed by surgery and treatment with radioactive iodine.

The main indication for this thyroglobulin dosage is therefore the monitoring of patients who have undergone thyroidectomy for thyroid carcinoma. Apart from this monitoring of thyroid cancers, measuring thyroglobulin levels is of little clinical utility, compared to other regular thyroid function tests such as TSH, T4 and T3.

However, another indication for thyroglobulin dosage exists: it is hypothyroidism in newborns. In fact, thyroglobulin then makes it possible to differentiate agenesis (absence of the thyroid), for which thyroglobulin is zero, from ectopia (abnormal position of the thyroid), for which thyroglobulin is detected.

On the other hand, it is also necessary to know that the levels of thyroglobulin rise, and remain increased during several days, following a palpation of the thyroid, and especially after a fine needle aspiration (removal of cells carried out by a the thin needle through the skin) of cysts or nodules.

Thyroglobulin analysis is performed by blood sample. A venous blood sample is thus taken from the elbow crease. No special preparation is necessary before this sample, which does not require you to be fasting.

However, some patients may be asked to stop taking thyroid hormone replacement before the examination. The purpose of this “hypothyroidism” is to stimulate the few very small amounts of tumor thyroid cells that may remain.

On the other hand, it is also possible in certain cases to carry out what is called a “thyrogen test”, that is to say an injection of the recombinant TSH which aims to stimulate the production of thyroglobulin. . 

Before proceeding with the analysis, care must therefore be taken to strictly follow the recommendations of the prescribing physician.

One technique routinely used for thyroglobulin analysis is chemiluminescence. The test measures the amount of thyroglobulin that is present in the blood. Chemiluminescence is a phenomenon that uses the ability of certain molecules, when brought to an excited state during a chemical reaction, to return to their ground state by restoring all or part of the energy in the form of an emission. from light.

Finally, in order to provide more information, it is possible to perform several thyroglobulin assays over a long period of time, which will therefore be more complete than a single isolated assay.

In patients in whom the thyroid has been removed from the thyroid gland due to cancer, the concentration of thyroglobulin in the blood should normally drop to undetectable levels.

If thyroglobulin levels are undetectable soon after surgery, but then increase again over time, the cancer may come back. It will then be necessary to perform a thyroid scintigraphy, which will confirm or not the recurrence.

In fact, when the results indicate that the thyroglobulin level is less than 1 ng / ml, and that in addition there are no anti-thyroglobulin antibodies present in the blood and the ultrasound is normal, the patient is in remission from her thyroid cancer. Lifelong follow-up will be maintained, with in particular a blood test taken each year for at least ten years, or even for life.

It is advisable, when performing a thyroglobulin dosage, to prescribe at the same time that of the anti-thyroglobulin antibodies. Indeed, these antibodies, proteins produced by the immune system against thyroglobulin, can alter the results of the thyroglobulin assay, making them unusable. And when thyroglobulin cannot be assayed due to the presence of these antibodies, it is therefore these anti-thyroglobulin antibodies themselves that will be evaluated for the follow-up of follicular thyroid cancer recurrence.

In fact, about one in five patients with thyroid cancer (or 20%) have these anti-thyroglobulin autoantibodies.

In addition, it should be noted that replacement therapy with thyroid hormone (thyroxine) should be followed, for life, by patients who have undergone total thyroidism removal, and also by most of those who have undergone subtotal thyroidectomy. .