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Background  Autosomal dominant non-autoimmune hyperthyroidism (ADNAH) is a rare genetic disorder of the endocrine system. Molecular genetic studies in ADNAH have revealed heterozygous germline mutations in the TSHR. To data, mutations leading to an increase in the constitutive activation of the TSHR have been described in the transmembrane segments, exoloops and cytoplasmic loop of TSHR. These mutations result in constitutive activation of the G αs /cAMP or G αq/11 /inositol phosphate (IP) pathways, which stimulate thyroid hormone production and thyroid proliferation.    Methodology/Principal Findings  In a previous study, we reported a new TSHR mutation located in the C-terminal domain of TSHR, which results in a substitution of the conserved Ile 691  for Phe. In this study, to address the question of whether the I691F mutated receptor could be responsible for G αs /cAMP or G αq/11 /IP constitutive activity, wild-type and TSHR mutants were expressed in COS-7 cells to determine cAMP constitutive activity and IP formation. Compared to the cell surface with expression of the A623V mutated receptor as positive control, the I691F mutated receptor showed a slight increase of cAMP accumulation. Furthermore, I691F resulted in constitutive activation of the G αq/11 /IP signaling pathway.    Conclusions/Significance  Our results indicate that Ile 691  not only contributes to keeping TSHR inactive in the G αs /cAMP pathways but also in the G αq/11 /IP cascade.

Constitutive Activation of the Thyroid-Stimulating Hormone Receptor (TSHR) by Mutating Ile691 in the Cytoplasmic Tail Segment