Association between Serum TSH Concentration and Body Mass Index in Euthyroid Subjects: The Role of Smoking

Data regarding the influence of minor changes in thyroid function on body mass index (BMI) are scarce and conflicting. This study aims to evaluate the association between normal levels of serum TSH and BMI, taking into consideration the effect of smoking tobacco. In this cross-sectional community-based survey, 1581 randomly selected subjects who had no histories of thyroid disorders were studied within the framework of the Tehran Lipid and Glucose Study. Serum TSH and TPOAb were measured from the fasting serum samples. Weight and height were measured and BMI was calculated. TSH concentrations within the reference range were positively associated with BMI (P < 0.002). Each unit increase of 1 µU/ml in TSH was associated with an increase in mean BMI of 0.31 kg/m2 (95% CI: 0.1 – 0.5, P < 0.002), an association which remained significant after adjustments for sex, physical activity, and smoking and in the subgroup with negative thyroid autoimmunity. The association between BMI and TSH remained significant only in nonsmokers (P < 0.004). The results suggest that a significant positive association exists between TSH and BMI in euthyroid nonsmokers.