Differences in Thyroid Autoimmunity and Thyroid Function Tests Between Individuals with and without Obesity: Is There a Correlation with Obesity Degree?

Background: Obesity, a rapidly escalating global health concern, is associated with comorbidities and chronic inflammation. However, the link between obesity and thyroid autoimmunity remains unclear.

Objective: This case-control study, conducted at a tertiary care center,aimed to elucidate the relationship between obesity and the degree of obesity, thyroid autoimmunity, and TFTs in euthyroid individuals with a BMI >30 kg/m2 and explore variations based on the degree of obesity.

Methods: Free thyroid hormones, TSH, thyroid peroxidase antibodies (anti-TPO), anti-thyroglobulin antibodies (Anti-Tg), and metabolic parameters (glucose, lipid profile, insulin resistance, hemoglobin A1c) were measured in 164 euthyroid patients with obesity and 73 lean subjects aged 18-65 years. Subjects with obesity were stratified into three groups based on body mass index (BMI): first-degree obesity (BMI 30-34.9 kg/m2), second-degree obesity (BMI 35-39.9 kg/m2), and third-degree obesity (BMI ≥ 40 kg/m2).

Results: The prevalence of thyroid antibody positivity was significantly higher in the obese group compared with the non-obese group, specifically for anti-TPO (45 [27.4%] vs. 7 [9.6%]) and anti- Tg (35 [21.3%] vs. 5 [6.8%]). Anti-Tg titers were elevated in the obese group (p=0.006), but anti- TPO levels were similar across the groups. Among the BMI-stratified groups, individuals with first and second-degree obesity exhibited higher anti-TPO positivity and anti-Tg titers compared with the control group. No significant differences were found in the third-degree obesity group. TSH and fT4 levels were higher in the obese group compared with the non-obese group (p=0.016 and p=0.045, respectively), whereas fT3 levels and the fT3/fT4 ratio remained consistent across the groups. Although no direct correlation was found between thyroid autoantibodies and metabolic parameters, individuals positive for anti-TPO and/or anti-Tg exhibited worse metabolic profiles compared with individuals who were antibody-negative.

Conclusion: There is an increase in thyroid autoimmunity among euthyroid individuals with obesity; however, this increase does not appear to be proportional to BMI. The effect of antibody presence on metabolic parameters in individuals with obesity is not yet fully understood.