The thyroid gland secretes primarily tetraiodothyronine (T), and some triiodothyronine (T). Under normal physiological circumstances, only one-fifth of circulating T is directly released by the thyroid, but in states of hyperactivation of thyroid-stimulating hormone receptors (TSHRs), patients develop a syndrome of relative T toxicosis. Thyroidal T production results from iodination of thyroglobulin (TG) at residues Tyr and Tyr, whereas thyroidal T production may originate in several different ways. In this study, the data demonstrate that within the carboxyl-terminal portion of mouse TG, T is formed independently of deiodination from T We found that upon iodination , T formation in TG was decreased in mice lacking TSHRs. Conversely, T that can be formed upon iodination of TG secreted from PCCL3 (rat thyrocyte) cells was augmented from cells previously exposed to increased TSH, a TSHR agonist, a cAMP analog, or a TSHR-stimulating antibody. We present data suggesting that TSH-stimulated TG phosphorylation contributes to enhanced T formation. These effects were reversed within a few days after removal of the hyperstimulating conditions. Indeed, direct exposure of PCCL3 cells to human serum from two patients with Graves' disease, but not control sera, led to secretion of TG with an increased intrinsic ability to form T upon iodination. Furthermore, TG secreted from human thyrocyte cultures hyperstimulated with TSH also showed an increased intrinsic ability to form T Our data support the hypothesis that TG processing in the secretory pathway of TSHR-hyperstimulated thyrocytes alters the structure of the iodination substrate in a way that enhances T formation, contributing to the relative T toxicosis of Graves' disease.

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