Thyroglobulin Contribution to T3-toxicosis Arises from Hyperstimulated Thyrocytes

CINTIA E. CITTERIO; BALAJI VELUSWAMY; STEPHEN ATKINS; TERRY SMITH; J PAUL BANGA; PETER ARVAN

Denver

Congreso; 86th Annual Meeting of the American Thyroid Association, Estados Unidos; 2016

Introduction: Thyroid hormone (TH) synthesis is initiated within the precursor protein thyroglobulin (TG), a large (2746 residue) glycoprotein synthesized in thyrocytes and post-translationally iodinated on multiple monoiodotyrosine (MIT) and diiodotyrosine (DIT) residues. Chemical coupling of DIT-DIT, or MIT-DIT, within the TG protein, initiates formation of thyroxine (T4) and tri-iodothyronine (T3), respectively. TH synthesis is tightly regulated by Thyroid Stimulating Hormone (TSH) interaction with its receptor (TSH-R). Two common thyroid diseases ? hypothyroidism caused by iodine deficiency, and autoimmune hyperthyroidism (Graves? disease) ? share a basic mechanism that includes massive hyperstimulation of TSH-Rs, which has been reported to alter the structure of TG in ways that may favor increased T3 production within TG during the initial iodination and coupling reactions (i.e., prior to any deiodination reactions). Methods / Case Presentation: To determine whether the structure of the TG may be altered to favor T3 formation, we treated PCCl3 thyrocytes with differing concentrations of TSH and collected the secreted TG. We then performed iodination in vitro followed by Western blotting with anti TG antibodies (to quantify the amount of secreted TG) as well as monoclonal anti-T3 antibodies (to identify de novo T3 formation directly within the intact TG protein). Additionally, we added a TSH-R stimulating antibody (KSAb1) that mimics the TSI of Graves? disease. To validate our method, we analyzed new T3 formation within TG from mice hyperstimulated by TSH as well as mice with genetic deletion of TSH-R. Results / Discussion: In vitro T3 formation within the TG band (and normalized to the amount of TG) from mice hyperstimulated by TSH was significantly higher than from mice with genetic deletion of TSH-R. Similarly, in PCCl3 thyrocytes that underwent hyperstimulation of TSH-Rs (either because high TSH or KSAb1 was added), T3 formation within the TG band (and normalized to the amount of TG) was notably increased. Conclusions: Altogether, we found that hyperstimulated thyrocytes produce a TG that is intrinsically more capable of de novo T3 formation. It seems possible that the structure of TG in patients with Graves? disease may be similarly altered to favor T3 formation.