Free thyroxine is an independent predictor of subcutaneous fat in euthyroid individuals

Abstract

ObjectiveThyroid function parameters have been associated with obesity, but associations with the type of adiposity have not been examined. We used ultrasound (US) to assess regional adiposity and investigated associations of thyroid function with parameters of central obesity.DesignCross-sectional study.MethodsA total of 303 apparently healthy individuals (age 42.9±8.8, body mass index (BMI) 19.0–43.3, median 26.2 kg/m2, 181 women) were examined for indices of the metabolic syndrome. BMI, waist and hip circumference, abdominal subcutaneous fat (SF), and preperitoneal fat (PF) layer was estimated. TSH, free thyroxine (fT4), triiodothyronine (T3), thyroid autoantibodies, insulin, glucose, and lipid levels were measured. Subjects receiving T4(9.2%) were excluded.ResultsSF and SF/PF ratio were inversely correlated with fT4levels (r=−0.169,P=0.023,r=−0.193,P=0.009 respectively). In multivariate analysis, fT4was a predictor of SF and SF/PF, independently of age, sex, and smoking. SF correlated with TSH levels (r=0.149,P=0.037). PF and SF were positively associated with T3levels (r=0.245,P=0.004 andr=0.189,P=0.019 respectively). T3levels were positively associated with BMI (r=0.257,P=0.0004), waist perimeter (r=0.324,P<0.0001), and waist-to-hip ratio (WHR;r=0.363,P<0.0001). The T3/fT4ratio was positively correlated with SF (r=0.182,P=0.028), WHR (r=0.267,P=0.0003), and BMI (r=0.146,P=0.043).ConclusionsIncreasing SF accumulation as assessed by US is associated with lower fT4and higher TSH levels among euthyroid slightly overweight individuals. These associations indicate that subtle variation in thyroid function may participate in regional adiposity.