Background: Constitutively active thyrotropin receptor (TSHR) mutations are the most common etiology of non-autoimmune hyperthyroidism (NAH). Thus far, the functionality of these mutations has been tested in vitro, but the in vivo models are lacking.

Methods: To understand the pathophysiology of NAH, the patient-derived constitutively active TSHR D633H mutation was introduced into the murine Tshr by homologous recombination.

Results: In this model, both subclinical and overt hyperthyroidism was observed, depending on the age, sex, and genotype. Homozygous mice presented hyperthyroidism at two months of age, while heterozygous animals showed only suppressed thyrotropin. Interestingly, at six months of age, thyroid hormone concentrations in all mutant mice were analogous to wild-type mice, and they showed colloid goiter with flattened thyrocytes. Strikingly, at one year of age, nearly all homozygous mice presented large papillary thyroid carcinomas. Mechanistically, this papillary thyroid carcinoma phenotype was associated with an overactive thyroid and strongly increased stainings of proliferation-, pERK-, and NKX2-1 markers, but no mutations in the “hot-spot” areas of common oncogenes (Braf, Nras, and Kras) were found.

Conclusions: This is the first study to reveal the dynamic age-, sex-, and genotype-dependent development of NAH. Furthermore, the study shows that a constitutively active TSHR can trigger a malignant transformation of thyrocytes.