Thyroglobulin (Tg) is a 660 kDa, dimeric protein produced by and used entirely within the thyroid gland. In earlier literature, Tg was referred to as "colloid".
Thyroglobulin should not be confused with Thyroxine-binding globulin, a carrier protein responsible for carrying the thyroid hormones in the blood.
Tg is used by the thyroid gland to produce the thyroid hormones thyroxine
(T4) and triiodothyronine
(T3). The active form of thyroxine, triiodothyronine, is produced both within the thyroid gland and periphery by 5'-deiodinase (which has been referred to as tetraiodothyronine 5' deiodinase
Tg is produced by the thyroid epithelial cells, called thyrocytes, which form spherical follicles. Tg is secreted and stored in the follicular lumen.
Via a reaction with the enzyme thyroperoxidase, iodine is covalently bound to tyrosine residues in thyroglobulin molecules, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT).
- Thyroxine is produced by combining two moieties of DIT.
- Triiodothyronine is produced by combining one molecule of MIT and one molecule of DIT.
Proteases in lysosomes digest iodinated thyroglobulin, releasing T3 and T4.
Patients with Hashimoto's thyroiditis
or Graves' disease
, frequently develop antibodies against Tg. Tg-specific antibodies help in the diagnosis of these diseases, but they also may be present in apparently healthy euthyroid
Thyroglobulin levels in the blood can be used as a tumor marker for certain kinds of thyroid cancer. Thyroglobulin levels in the blood can also be elevated in cases of Graves' disease.