Body mass index (BMI) is associated with serum thyroid-stimulating hormone (TSH) level in infertile women: a cross-sectional study

Studies suggest that there is a relationship between body mass index (BMI) and thyroid-stimulating hormone (TSH) levels. But conflicting evidence exists regarding the relationship between the two variables. Moreover, thyroid function is closely related to female fertility and has certain effects on infertility. Therefore, the present study will explore the relationship between BMI and TSH levels in patients with infertility in our center. We retrospectively analyzed relevant indicators of 2,789 in Tubal Factor Infertility patients undergoing assisted reproduction technology from January 2016 to December 2018 in our center in order to analyze the relationship between BMI and serum TSH level. The medical histories of patients were reviewed. The relationship between BMI and TSH was assessed using smooth curve fitting and multivariate regression model. The smoothing curve fitting between BMI and TSH exhibited a non-linear relationship, and the resulting curve exhibited a two-stage change and a breakpoint. By multivariate piecewise linear regression, we found that the TSH level was increased with the increase of BMI when the BMI was greater than 25.3 kg/m(2) (beta 0.06, 95% CI 0.02, 0.01; p = 0.0028). In contrast, the TSH level was decreased with the increase of BMI when the BMI was less than 25.3 kg/m(2) (beta -0.02, 95% CI -0.05, 0.00; p = 0.0573). Collectively, our study described a non-linear relationship between BMI and TSH level in infertile patients after adjustment of potential confounders. However, such causal relationship between BMI and TSH in infertile women still needs to be further clarified in future investigations.