The Relationship between Thyroid Function and Different Obesity Phenotypes in Korean Euthyroid Adults

Background: Thyroid disease and metabolic syndrome are both associated with cardiovascular disease. The aim of this study was to investigate the correlation between thyroid hormones and obesity sub-phenotypes using nationwide data from Korea, a country known to be iodine replete. Methods: This study was based on data obtained from the sixth Korea National Health and Nutrition Examination Survey, administered from 2013 to 2015. A total of 13,873 participants aged >= 19 years were included, and classified into four groups: metabolically healthy non-obesity (MHNO), metabolically healthy obesity (MHO), metabolically unhealthy non-obesity (MUNO), and metabolically unhealthy obesity (MUO) by body fat on the basis of body mass index and metabolic health. Results: At baseline, serum free thyroxine (fT4) values were significantly higher in the MHNO phenotype (MHNO, 1.27 +/- 0.01 ng/dL; MHO, 1.25 +/- 0.01 ng/dL; MUNO, 1.24 +/- 0.01 ng/dL; MUO, 1.24 +/- 0.01 ng/dL, P < 0.001) in total study population. However, this significant association no longer remained after adjustment for age, urine iodine concentration, and smoking (P= 0.085). After adjustment for confounders, statistically significant association was observed between lower thyroid stimulating hormone (TSH) and MHNO phenotype (P= 0.044). In men participants (not women), higher fT4 values were significantly associated with MI INO phenotype (P<0.001). However, no significant association was observed between thyroid function (TSH or fT4) and obesity phenotypes in groups classified by age (cutoff age of 55 years). Conclusion: Although there was a difference by age and sex, we found that the decrease of TSH and the increase of fT4 values were associated with MHNO.