Recently, the prevalence of obesity and metabolic syndrome is rapidly increasing among children and adolescents in Korea. The prevalence of obesity in Korean children is estimated to be around 10%, and the incidence of metabolic syndrome was 30 to 40% in overweight or obese children and adolescents in Korea. Korea has been undergoing considerable changes in the lifestyle influencing on obesity and metabolic syndrome along with rapid economic growth. The lifestyle trends contributing to both of these phenomena include changes in dietary patterns and habits with western dietary pattern and high-fat intake, decreasing of physical activity, and increasing sedentary behaviors. Over-intake of nutrients induces fat deposition in liver and skeletal muscle as well as adipose tissue, and it can alter energy metabolism. We have recently observed that in cultured myotubes, induction of insulin resistance by exposure to saturated fatty acids reduced TR expression and DNA binding activity, and TRα1 expression was decreased in human type 2 diabetes mellitus. Thyroid hormone stimulates metabolic rate, protein catabolism, fatty acid oxidation and nuclear-encoded mitochondrial OXPHOS gene expression. We hypothesized that thyroid hormone mediated transcription may be reduced in insulin resistance induced by high fat diet (HFD), and thus may contribute to impaired expression of oxidative metabolic genes linked to insulin resistance in vivo. We have shown that HFD effects on subsets of T3-responsive genes in laboratory rats suggest that obesity associated with HFD may alter the pattern of coactivator/corepressors within the TR transcriptional complex. These results indicate that HFD induce intracellular T3 resistance, and then may exacerbate metabolic disturbance. Furthermore, T3 responsive genes are down-regulated in the liver of humans with obesity and diabetes. Since childhood obesity leads to the higher prevalence of obesity in adulthood, which increases the risk of metabolic syndrome, the medical profession must be actively involved with educating the parents and the community in order to raise their awareness of the importance of healthy life styles and dietary habits.