Acts as a substrate for the production of iodinated thyroid hormones thyroxine (T4) and triiodothyronine (T3) (PubMed:7021557). The synthesis of T3 and T4 involves iodination of selected tyrosine residues of TG/thyroglobulin followed by their oxidative coupling (PubMed:7021557). Following TG re-internalization and lysosomal-mediated proteolysis, T3 and T4 are released from the polypeptide backbone leading to their secretion into the bloodstream (By similarity). One dimer produces 7 thyroid hormone molecules (By similarity). Monomer (By similarity). Homodimer (via ChEL region); occurs in the endoplasmic reticulum and is required for export to the Golgi apparatus (By similarity). Homooligomer; disulfide-linked; stored in this form in the thyroid follicle lumen (By similarity). Secreted into the follicular lumina of the thyroid (PubMed:12387814). Localizes to colloid globules, a structure formed in the thyroid follicle lumen consisting of cross-linked TG arranged in concentric layers (By similarity). Expressed in thyroid epithelial cells. The cholinesterase-like (ChEL) region is required for dimerization and export from the endoplasmic reticulum. Iodinated on tyrosine residues by TPO (PubMed:12325367). There are 4 pairs of iodinated tyrosines used for coupling: acceptor Tyr-24 is coupled to donor Tyr-149 or Tyr-234, acceptor Tyr-2500 is coupled to donor Tyr-2467, acceptor Tyr-2690 in monomer 1 is coupled to donor Tyr-2690 in monomer 2 and acceptor Tyr-1241 in monomer 1 is coupled to donor Tyr-108 in monomer 2 (By similarity). Sulfated tyrosines are desulfated during iodination. Undergoes sequential proteolysis by cathepsins to release thyroxine (T4) and triiodothyronine (T3) hormones (By similarity). In the thyroid follicle lumen, cross-linked TG (storage form) is solubilized by limited proteolysis mediated by cathepsins CTSB and/or CTSL (By similarity). Partially cleaved TG is further processed by CTSK/cathepsin K and/or CTSL resulting in the release of T4 (PubMed:11082042). Following endocytosis, further processing occurs leading to the release of T3 and more T4 hormones (By similarity). Belongs to the type-B carboxylesterase/lipase family. The cholinesterase-like (ChEL) region lacks the Ser residue of the catalytic triad suggesting that it has no esterase activity.