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The clinical significance of thyroglobulin (Tg)
The clinical significance of thyroglobulin (Tg). The tumor tissue of medullary thyroid cancer is derived from thyroid C cells, not thyroid filter cell epithelial cells, so the serum Tg level of patients with this type of cancer does not increase or even decrease.
Thyroglobulin (Tg) is a macromolecular glycoprotein (MW = 660000) stored in the follicular gel of the thyroid. Thyroglobulin is the prohormone that synthesizes T4 and T3 inside the thyroid. The protease-containing lysosome cleaves T4, T3, and Tg to release T4 and T3. Thyroglobulin (Tg) is secreted by thyroid follicular epithelial cells and is the precursor protein and storage carrier of thyroid hormone synthesis. The increase in serum Tg levels is related to the following three factors: goiter; thyroid tissue inflammation and injury; TSH, hCG or TRAb stimulates the thyroid.
Thyroglobulin exists in the serum of healthy individuals, and its value will increase when some disorders that damage thyroid tissue occur. Hashimoto (Chronic lymphocytic thyroiditis CLT), Graves (viral diffuse goiter), thyroid adenoma, subacute thyroiditis, thyroid cancer, etc. can all have elevated circulating Tg levels. Therefore, it is mainly used as an indicator after thyroidectomy. Patients who have undergone thyroidectomy and have no antibodies to thyroglobulin in the blood can be tested for thyroglobulin.
Serum thyroglobulin concentration reflects thyroid mass, thyroid damage and TSH receptor activation. When the patient has a goiter or is in a state of most hyperthyroidism, the serum Tg increases. The abnormally elevated serum Tg concentration is due to abnormal thyroid mass, excessive stimulation of the thyroid, or physical thyroid injury secondary to surgery, fine needle aspiration, or thyroiditis. In the case of differentiated thyroid cancer (DTC), serum Tg concentration reflects thyroid mass (tumor or normal residual tissue), thyroid injury (surgery or FNA), and TSH receptor activation. Because TSH level is the main regulator of serum Tg concentration, it is difficult to explain the serum Tg value without knowing the patient’s TSH status.
Although there is no “normal Tg reference range” for DTC patients receiving treatment, the normal relationship between thyroid mass and serum provides an important reference point. When the serum TSH level is normal, 1g normal thyroid tissue can specifically release 1μg/L Tg into the circulation; if the serum TSH is inhibited below 0.1mU/L, it only releases 0.5μg/L Tg.
The clinical significance of elevated Tg are:
Elevated Tg concentration has been reported in different thyroid diseases, such as Hashimoto’s disease, Graves’ disease, etc. Tg can also help distinguish subacute thyroiditis from man-made thyrotoxicosis. The detection of Tg for congenital hypothyroidism can be used to distinguish between congenital hypothyroidism and thyroid hypoplasia or other pathological conditions.
Tg detection is mainly used for follow-up of patients after total or subtotal thyroidectomy. Since the thyroid is the only known source of Tg, it is successfully ablated with radioactive iodine after total or subtotal thyroidectomy.
After tissue, the serum Tg concentration will drop to very low, even undetectable. For patients with partial thyroidectomy, the detected Tg level depends on the amount of thyroid tissue remaining after the operation. If Tg is still detectable after total thyroidectomy, it may indicate residual or recurrence of DTC. Therefore, a significant increase in Tg often indicates a recurrence of the disease.
Use serum Tg
The limiting factor of the assay is the presence of Tg autoantibodies in the patient. These antibodies can interfere with the immunoassay of Tg, causing the value to rise or fall erroneously. For patients who need Tg measurement, it is very important to determine the level of Tg autoantibodies. The presence of serum thyroglobulin autoantibodies (TgAb) can interfere with the determination of thyroglobulin (Tg). Therefore, as long as it contains TgAb, even serum containing very low levels of TgAb cannot be used for Tg testing.
It should be noted that the tumor tissue of medullary thyroid cancer is derived from thyroid C cells, not thyroid filter cell epithelial cells, so this type of cancer
The serum Tg level of patients with disease does not increase or even decrease
(source:internet, reference only)