Altered Craniofacial and Bone Morphology in Hyperthyroid Knock-In Mice Carrying TSHR D633H and M453T Mutations.

BACKGROUND: Constitutively active mutations (CAMs) in the thyrotropin receptor (TSHR) are the major cause of nonautoimmune hyperthyroidism. TSHR is a key regulator of thyroid hormone synthesis, which is essential for skeletal formation, bone turnover, and craniofacial development. In addition to its role in thyroid hormone regulation, thyrotropin and its receptor have been proposed to independently influence bone formation and resorption. However, the specific effects of constitutively active TSHR mutations on cranial and skeletal development have not been investigated in mouse models. METHODS: Cranial morphometry, micro-computed tomography, and three-point bending tests were performed in established TSHR knock-in mouse models carrying patient-derived TSHR D633H or M453T CAMs. Homozygous TSHR D633H mice exhibit mild, transient hyperthyroidism at 2 months of age, more pronounced in females, whereas homozygous TSHR M453T mice develop a more severe, iodine-dependent hyperthyroid phenotype. RESULTS: Both TSHR CAM lines showed altered craniofacial morphology, particularly in nasal bone dimensions, resulting in a shorter snout compared with wild-type (WT) controls. The incidence of malocclusion was significantly increased in both homozygous and heterozygous mice, regardless of sex. TSHR D633H mice displayed no significant changes in femoral or tibial bone structure or biomechanical properties. In contrast, TSHR M453T mice exhibited hyperthyroidism-dependent alterations in trabecular bone mineral density (BMD) and architecture, while cortical bone was unaffected. Body and tail lengths were unchanged in TSHR D633H mice. M453T homozygous mice had reduced tail length at weaning in an iodine-dependent manner, which normalized with age. CONCLUSIONS: study demonstrates that TSHR CAM-induced hyperthyroidism alters craniofacial morphology and increases the incidence of malocclusion in mice. Structural changes and altered BMD in M453T mice were dependent on the severity of hyperthyroidism. These findings highlight the role of TSHR signaling and thyroid status in craniofacial and skeletal development, warranting further mechanistic investigation.