Associations Of Obesity And Its Metabolic Phenotypes With Serum Thyroid Hormones

Objective:The obesity-related cardiovascular and metabolic risks are largely dependent on body fat distribution.Leg fat has protective role in metabolism,while trunk and abdominal fat accumulation significantly increases metabolic risks.Obese individuals have different metabolic phenotypes,whether the metabolically healthy obese phenotype really a healthy status is still unclear.Thyroid hormones exert pivotal roles in the regulation of substance and energy metabolim.The present study aimed to explore the associations of segmental and abdominal fat distribution with serum thyroid hormones,and investigate characteristics of thyroid hormones in different metabolic phenotypes of obesity.Methods:The study recruited euthyroid population in Shanghai communities.Bioelectrical impedance analysis was used to measure total fat and segmental fat(arm,trunk,and leg)distribution.Abdominal subcutaneous fat area(SFA)and visceral fat area(VFA)were precisely measured by magnatic resonance imaging.Serum free triiodothyronine(FT3),free thyroxine(FT4)and thyroid stimulating hormone(TSH)were detected by electrochemical luminescence immunoassay.Fat percentage(fat%)?25%for men and?30%for women were used as cutoffs for obesity.The simple obesity index—body mass index(BMI)?25 kg/m~2was used to define overweight/obesity.According to the 2017 diagnostic criteria for metabolic syndrome by the Chinese Diabetes Society,metabolically unhealthy was defined as two or more components(waist circumference was excluded)of metabolic syndrome.Results:(1)In 891 community population(394 middle-aged and elderly men and 497postmenopausal women),none of the segmental fat parameters were significantly correlated with thyroid hormones in men(all P>0.050).In postmenopausal women,the leg fat mass to trunk fat mass ratio was significantly and negatively related to FT3(all P<0.050),leg fat was also negatively related to FT3(all P<0.050),while trunk fat was positively related to FT3(all P<0.050).(2)In 1036 community population(445 men and 591 women),SFA was significantly increased with the increase of FT3and FT3/F4 tertiles groups in both men and women(all P<0.050),while VFA was not significantly changed(all P>0.050).Multiple stepwise regression analysis showed that SFA was an independent and positive influencing factor of FT3 in both men and women(all P<0.050),and SFA was further independently and positively related to FT3/FT4 in men(all P<0.050).VFA was not related to thyroid hormones(all P>0.050).(3)In 1023 community population(437 men and 586 women),according to obesity based on fat%and overweight/obesity based on BMI,subjects were divided into metabolically healthy non-obese,metabolically unhealthy non-obese,metabolically healthy obese and metabolically unhealthy obese phenotypes.Regardless of using definitions based on fat%or BMI,FT3 and FT3/FT4were all significantly and positively related to the metabolically healthy obese phenotype and the metabolically unhealthy obese phenotype(all P<0.050).In addition,FT4 was negatively related to the metabolically unhealthy obese phenotype(all P<0.050),TSH was postitively related to the metabolically unhealthy non-obese phenotype(all P<0.010).Conclusions:In euthyroid population,leg fat was negatively and trunk fat was positively related to serum FT3 levels in postmenopausal women,suggesting that higher FT3 within the euthyroid range was related to advese fat distribution.Higher FT3 and FT3/FT4 within the euthyroid range were mainly contributed by abdominal subcutaneous fat accumulation instead of visceral fat accumulation.Both the metabolically healthy and unhealthy obese phenotypes were characterized by increased FT3 and FT3/FT4,suggesting that the metabolically healthy obese phenotype may not be a stable and benign status.