Association between Thyroid Function and Body Mass Index in Tehran Thyroid Study Population

Introduction: The prevalence of overweight and obesity is alarmingly rising around the world. With the increasing prevalence of obesity, study of thyroid disorders, caused by an abnormal increase in the body mass index (BMI), is becoming more and more important. This study aimed to evaluate the association between increased BMI and thyroid hormones among Iranians participating in the Tehran Thyroid Study. Materials and Methods: This cross-sectional study was conducted on 3079 female and 2274 male participants, who were categorized into two groups: one with abnormal BMI>25 kg/m2 and the other with normal BMI<25 kg/m2. Thyroid hormone levels were also evaluated and compared. The linear regression analysis and Spearman's coefficient test were used to determine the correlations between BMI and thyroid hormones. The odds ratio (OR) and 95% confidence interval (CI) were measured to study the impact of obesity on thyroid function. Results: BMI had a positive correlation with the thyroid-stimulating hormone (TSH) level (r=0.04, 95% CI: 0.01, 0.07; P<0.05) and a negative correlation with the free thyroxine (FT4) level (r=-0.18, 95% CI: -0.21, -0.16; P<0.05). Also, BMI above 25 kg/m2 was associated with a higher OR of clinical hypothyroidism (OR=1.71, CI: 1.04, 2.79; P<0.05). No significant association was observed between abnormal BMI and other thyroid dysfunctions. Also, no significant association was found between an abnormal increase in BMI and TPOAb positivity in the adjusted results. Conclusion: This study showed that an abnormal increase in BMI was associated with altered thyroid functions. Consequently, weight control and reduction can be suggested to individuals with overweight and obesity to improve their thyroid function.