Thyroglobulin (Tg) is a glycoprotein with a molecular weight of approx. 660 kD. It basically consists of two protein chains of 300 kD and 330 kD, linked together by disulfide bridges.
Tg is synthesized in large quantities by the thyrocytes and released into the lumina of the thyroid follicles. Production of Tg is stimulated by TSH, intrathyroidal iodine deficiency and the presence of thyroid‑stimulating immunoglobulins.
Tg plays a decisive role in the synthesis of the peripheral thyroid hormones T3 and T4. It contains about 130 tyrosine residues, some of which can be iodinated to monoiodo‑ and diiodothyrosine (MIT and DIT) in the presence of TPO (thyroperoxidase) and iodide. 3 The subsequent coupling of MIT and DIT to form T3 and T4 also takes place on the Tg‑matrix with the involvement of TPO.
During Tg‑synthesis by the thyrocytes and the transport of Tg to the follicles, small quantities of the protein can pass into the bloodstream. Accordingly, low concentrations of Tg can also be found in the blood of persons not suffering from thyroid diseases. Low levels of circulating Tg therefore indicate the presence of thyroid tissue. Following successful total thyroidectomy, Tg is no longer detectable.
In cases of congenital hypothyroidism the determination of Tg can be used to distinguish between the complete absence of the thyroid gland and thyroid hypoplasia or other pathological conditions.
On the other hand, injury to the follicle wall can result in larger quantities of Tg passing into the blood. Tg is therefore regarded in particular as a marker for the morphological integrity of the thyroid gland.
The determination of Tg can also be useful in distinguishing between subacute thyroiditis and factitious thyrotoxicosis. In the latter case, low Tg values are to be expected due to the suppression of TSH. The presence of antibodies to Tg can falsify the result of a Tg determination.