tg (Thyroglobulin)-Its Role in Thyroid Cancer

What is tg (Thyroglobulin) ?

Thyroglobulin is a glycoprotein. It is the precursor to the thyroid hormones which are T3 (triiodothyronine) and T4 (tetraiodothyronine).

This glycoprotein constitutes a molecular mass of 660 kD. This has 2 identical subunits, but the complete hydrolysis of this protein generates only 2 to 4 molecules of both T3 and T4.

The Thyroglobulin protein has a 19-amino acid signal peptide accompanied by 2,748 residues. The sequence of this protein is 77% similar to that of the bovine protein. (Dumont et al., 1989).

Cytogenetic location

Tg is located on the chromosome 8q24.22 

Structure of Thyroglobulin

Thyroglobulin being the protein precursor of thyroid hormones, which are essential for growth, development and the control of metabolism.

the process of thyroid hormone synthesis starts from TG which happens in the thyroid gland through a process called as iodination accompanied by the coupling of pairs of tyrosines, later completed by TG proteolysis.

The hormonogenic tyrosines has not not yet been clearly identified although the coupling reaction can be enabled due to close proximity of the Tyrosine within TG.

The mechanistic understanding cannot be sought due to the lack of a 3D structure of Thyroglobulin.

Thyroglobulin is synthesized exclusively by thyroid follicular cells and serves as the scaffold for thyroid hormonogenesis. 

What is the role of Diagnostic Modalities of Thyroglobulin in Thyroid Cancer

As thyroid tissue is the only source of circulating Thyroglobulin, the thyroglobulin test is the cornerstone of post-operative biochemical monitoring for recurrence in patients with follicular-derived (differentiated) thyroid cancer.

What are the methods of detection of Thyroglobulin

  • Radioimmunoassay (RIA)
  • Immunometric assay (IMA)
  • Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS)

The Thyroglobulin autoantibodies that are present in approximately 25% of differentiated) thyroid cancer patients remain a persistent problem.

This causes interference with primarily the Immunometric assay class of methods of detection of Thyroglobulin , but also potentially influencing the clinical utility of Radioimmunoassay and Liquid Chromatography Tandem Mass Spectrometry measurements by in-vitro and/or in-vivo mechanisms.