Whilst the majority of differentiated thyroid cancers (DTC) have oncogenic mutations, a significant minority may be driven by the overexpression of proto-oncogenes. PTTG and PBF are proto-oncogenes which are induced in DTC, elicit tumours in xenograft models and interact in vitro, where PBF shuttles PTTG into the nucleus. However, the relative contributions of each gene to DTC has not been delineated. Here, we constructed a bi-transgenic murine model over-expressing both PBF and PTTG specifically in the thyroid gland (PTTG/PBF-Tg), and characterised it in comparison to age and sex matched single transgenic (PTTG-Tg, PBF-Tg) and wild-type mice. A total of 68 wild-type, 98 PBF-Tg, 25 PTTG-Tg and 23 PTTG/PBF-Tg mice were assessed. No significant difference in thyroid weight was observed between male and female mice within each of the four genotypes. However, there was a significant 2.7-fold increase in thyroid weight, adjusted for total body weight, in bi-transgenic mice compared to wild-type (P<0.001) at 6 weeks of age. PBF-Tg thyroid weight was 1.7-fold higher than wild-type mice (P<0.001), whereas PTTG thyroid weights were similar to wild-type (0.95-fold; P=0.18). Interestingly, bi-transgenic thyroids were 1.6-fold heavier than PBF-Tg thyroids (P<0.001). Enlarged thyroid growth in bitransgenic mice was accompanied by significant hyperplasia and macrofollicular lesions. As oncogenic expression of PTTG is known to induce genetic instability (GI), we determined GI levels through FISSR-PCR in primary thyroid cultures of each genotype. Compared with WT mice (arbitrary GI index=0%), PBF-Tg mice had a GI index of 19.8±1.8%; PTTG-Tg mice of 7.6±1.6%; and PTTG-Tg/PBF-Tg mice of 37.9±2.7%. Together, our data reveal a complex interplay between PTTG and its binding partner PBF in vivo; the bi-transgenic thyroid phenotype is closer to that of PBF-Tg mice than PTTG-Tg mice, but reveals increased goitre size and heightened genetic instability than either single transgenic model alone.