Thyroglobulin (precursor for thyroid hormone synthesis) is a large secreted glycoprotein comprising contiguous region I (multiple type-1 repeating units engaging the first ϳ1,191 residues, followed by a ϳ245-residue hinge region), regions II-III (multiple type-2 and 3 repeating units, comprising ϳ720 residues), and the C-terminal cholinesterase-like (ChEL) domain (ϳ570 residues). A signal peptide attached to ChEL makes an independent secretory protein that binds to I-II-III, stabilizing it and rescuing the secretion of I-II-III that would otherwise be trapped in the endoplasmic reticulum (ER). In this study, we found that a signal peptide attached to regions II-III also makes for an efficient secretory protein that neither demonstrably interacts nor has its secretion enhanced by the presence of secretory ChEL. By contrast, region I, either with or without the hinge region, cannot be secreted on its own and remains in the ER where it is bound to ER chaperones BiP and GRP94. Whereas ChEL can rescue secretion of I-II-III, it can rescue I-II only very weakly, and region I not at all. Yet, ChEL begins to rescue region I in cells that also co-express secretory II-III. The data suggest that conformational maturation of region I is a limiting step in the thyroglobulin maturation process, and this step is facilitated by the presence of both regions II-III and ChEL. Mutations causing hypothyroidism might induce solely local/regional misfolding or may interfere more globally by impeding interactions between regions that are required for thyroglobulin secretion.In vertebrates, thyroid hormones are first created in the apical luminal cavity of follicles of the thyroid gland, upon iodination of the thyroglobulin (Tg) 3 protein (1, 2). It is the structure of Tg (3, 4) rather than the specificity of the peroxidase (5) that is thought to catalyze the coupling of di-iodotyrosyl residues 5 and 130 at the N terminus of Tg (6, 7), forming thyroxine within the Tg molecule. Even in the endostyle of protochordates (the earliest species in which thyroid hormone synthesis is known to occur), a Tg-like protein has been reported (8), whereas no other thyroidal proteins have yet been discovered to substitute effectively for Tg in this role.The primary sequence of Tg (ϳ2,746 residues in mouse Tg after signal peptide cleavage) comprises disulfide-rich contiguous regions: region I (multiple type-1 repeating units engaging the first ϳ1,191 residues followed by a ϳ245-residue hinge region); II-III (multiple type-2 and type-3 repeating units, spanning ϳ720 residues); and the C-terminal cholinesterase-like (ChEL) domain (ϳ570 residues). The genetic origins of these units are still being discovered, but the type-1 repeat is a structural motif that is appears in evolution as a protease inhibitor (9); the type-2 and type-3 repeats have limited homology to other proteins; whereas ChEL exhibits homology to acetylcholinesterase (10 -12). Because no crystallographic data are yet available, the precise physical relationships between the Tg regions remain ...