Obesity is associated with increased serum TSH level, independent of thyroid function

To reinvestigate the relationship between circulating TSH levels and adiposity in a cohort of obese people, who have normal thyroid function.

Retrospective cross-sectional analysis was carried out on 226 euthyroid obese or overweight female patients. Thirty-nine female lean and euthyroid subjects (BMI<25 kg/m2) were included in the study group. TSH, free thyroxine (FT4), free triiodothyronine (FT3), fasting plasma levels of insulin and glucose, homeostasis model assessment (HOMA) for insulin resistance (HOMA-IR) and insulin secretion (HOMA-b cell), body weight, height, body mass index (BMI) and waist circumference were assessed.

Serum TSH levels were higher in the obese than in the lean subjects. In the study group (lean and obese subjects), there was a significant positive correlation between serum TSH and body weight (r=0.231, p<0.001), BMI (r=0.270, p<0.001), waist circumference (r=0.219, p=0.001), fasting insulin (r=0.201, p=0.002) and HOMA-IR (r=0.201, p=0.002); there was no correlation between serum FT4 and any of the parameters. A multivariate linear regression analysis revealed that only BMI (p=0.012, 95% CI=0.01-0.08) contributed significantly to the variance of TSH.

This study strongly supports existing, but contradictory evidence that serum TSH levels are positively correlated with the degree of obesity and some of its metabolic consequences in overweight people with normal thyroid function.