Free T4 is negatively correlated with body mass index in euthyroid women

Abstract

Background/Aims: Overt thyroid dysfunction is well known to affect weight. However, the influence of normal-range changes in thyroid status on body mass index (BMI) is unclear. We sought to evaluate thyroid function (free T4, TSH) and its possible relationship with BMI and lipid profiles in euthyroid subjects. Methods: A total of 1572 euthyroid women (mean age 46.2 years) who visited Daegu Catholic University Medical Centre for primary health screening participated in this cross-sectional study. Women who were not euthyroid and women who took thyroid medication were excluded. TSH, free T4, and lipid profile [total-cholesterol, triglyceride (TG), HDL-C, LDL-C] were evaluated. Results: Obese euthyroid women had lower free T4 levels than did lean euthyroid women. After adjustment for age and smoking, free T4, but not TSH, was significantly negatively correlated with BMI. After adjustment for age, smoking, and BMI, free T4 was negatively correlated with TG to a significant degree. Conclusions: We demonstrated a negative correlation between free T4 within the normal range and BMI in euthyroid subjects. These findings suggest that low free T4 is associated with obesity in euthyroid subjects.