Trefoil factor protein 3 (Tff3) is a small peptide expressed mainly in goblet cells with a role in healing and protection of epithelium. Recent research suggests that the Tff3 protein also plays an important role in metabolic and neurodegenerative diseases. Metabolic diseases, such as Type 2 diabetes and neurodegenerative diseases, such as Alzheimer's disease, share many pathophysiological links. Studying these links in the early stages, before the onset of advanced symptoms, could contribute to a better understanding of the pathogenesis of these complex disorders and to the successful development of effective therapies. Therefore, the aim of this doctoral thesis was to investigate the role of Tff3 in the liver and the hippocampus in the early stages of pathological processes activation due to short-term treatment with a high-fat diet. First, a new congenic Tff3-/- mouse strain was developed on the C57BL/6N genetic background without additional relevant metabolic mutations. Subsequently, the newly developed Tff3-/- mice of both sexes and corresponding wild-type controls were fed a highfat diet for 9 weeks. We examined the general metabolic status of the animals and analyzed the expression of markers of relevant pathophysiological pathways in the liver and in the hippocampus. Tff3-/- animals had significantly lower weight and reduced fat accumulation in the liver compared with WT controls, suggesting a protective effect of Tff3 deficiency. The results of molecular analysis showed decreased levels of the Il1α and Cxcr7 genes in the liver of Tff3-/- males, whereas the levels of the Irs2 and Atf4 and Ybx1 and Anp32a were increased in Tff3-/- females. In the hippocampus of Tff3-/- males, the protein level of the marker of immature neurons (Dcx) and the marker of microglia (Iba1) were decreased, whereas levels of Dcx and the marker of mature neurons (NeuN) were increased in Tff3-/- females. Analysis of neurogenesis markers shows more pronounced effect in the hippocampus of Tff3-/- females. No statistically significant differences in the aforementioned altered markers were detected in both liver and hippocampus of the animals exposed to a standard diet, suggesting that the observed changes are a consequence of stress induced by high-fat diet. This research provides new insights into the role of Tff3 protein in the liver and the hippocampus and provides direction for further investigations aimed at uncovering the precise molecular mechanisms. In addition, it highlights the need to include both sexes in future research, as the observed phenotype differs significantly by sex.