Associations Between Thyroid Hormones Within the Euthyroid Range and Indices of Obesity in Obese Chinese Women of Reproductive Age

Background: A number of researches have reported that thyroid hormones are associated with obesity. However, the relationship of serum levels of thyroid hormones in the normal range with obesity and parameters of obesity in women of childbearing age remains controversial. The purpose of this study was to examine serum levels of thyroid hormones within the normal range in obese Chinese women of reproductive age and to investigate the relationship between concentration of thyroid hormones and indices of obesity, including body mass index (BMI), waist-to-hip ratio (WHR), insulin resistance, blood glucose, blood lipids, and blood pressure. Methods: One hundred fifty-one obese women of reproductive age and 160 nonobese women of reproductive age were enrolled in this study. Serum levels of thyroid-stimulating hormone (TSH) of all subjects were within the normal reference range (0.35-4.94 mIU/L). The serum levels of free triiodothyronine (FT3), free thyroxine (FT4), and TSH, height, body weight, BMI, waist and hip circumferences, WHR, fasting blood glucose (FBG), fasting insulin (FI), homeostasis model assessment of insulin resistance (HOMA-IR), total triglycerides (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), systolic blood pressure (SBP), and diastolic blood pressure (DBP) were measured in all subjects. Quantile regression analysis was used to analyze the associations of serum levels of FT3, FT4, and TSH with values of BMI, WHR, FBG, FI, HOMA-IR, TG, TC, LDL-C, HDL-C, SBP, and DBP. Results: In the group of obese women, serum levels of FT4 were lower (P < 0.001) and serum levels of TSH were higher (P < 0.001) compared with nonobese controls. After adjusting for covariables, quantile regression analysis showed that serum levels of FT4 were inversely associated with BMI values between the quantile levels of 0.29 and 0.60 of BMI (i.e., BMI level of 22.49 and 28.31 kg/m(2), respectively). Meanwhile, we found that serum levels of TSH positively correlated with BMI values after the quantile level of 0.51 (i.e., BMI level of 27.06 kg/m(2)), positively associated with TC after the quantile level of 0.6 (i.e., TC level of 4.86 mM), and positively associated with LDL-C after the quantile level of 0.39 (i.e., LDL level of 1.96 mM). No significant associations were found between serum levels of thyroid hormones and values of WHR, FBG, FI, HOMA-IR, TG, HDL-C, SBP, and DBP. Conclusions: FT4 and TSH play an important role in regulating the weight in women with normal thyroid function during their reproductive years. Women with decreased serum FT4 or increased serum TSH levels have a higher risk of developing obesity. Besides, TSH has a significant influence on metabolism of blood lipids. Women with higher serum levels of TSH have a higher risk of incidence of lipid metabolism disorders.