Background: The aim of this study was to examine the relationship between thyroid hormones and smoking and several other parameters like age, gender, insulin, and anthropometric and metabolic parameters in subjects with a wide range of body mass index (BMI).
Patients and methods: A total of 931 euthyroid normal weight (BMI<25.0 kg/m2), overweight and obese subjects (BMI ≥25.0 kg/m2), 663 women and 268 men, aged 18-68 yr, were investigated. Fasting TSH, free T3 (FT3), free T4 (FT4), insulin, glucose, and lipid serum levels were determined. Waist circumference was measured as an indirect parameter of central fat accumulation.
Results: Smokers were younger (p<0.001) and showed higher FT3 (p<0.01), and triglyceride (p<0.01) levels and lower glucose (p<0.01) and HDL (p<0.001) concentrations than non smoking subjects. FT3 levels were directly associated with BMI (p<0.001), waist circumference (p<0.001), insulin (p<0.001), and triglyceride (p<0.01) levels and negatively correlated with age (p<0.001) and HDL-cholesterol levels (p<0.001). When a multiple regression analysis was performed with FT3 levels as the dependent variable, and smoking, age, gender, and TSH, insulin, triglyceride, and HDL-cholesterol serum concentrations as independent variables, FT3 levels maintained an independent positive association with smoking (p<0.05), age (p<0.001), male sex (p<0.001), waist circumference (p<0.05), and insulin levels (p<0.001).
Conclusions: Smoking increases FT3 levels independently of age, gender, obesity, body fat distribution and metabolic parameters.