Thyroid function and lipid profile in euthyroid adults: the TCLSIH cohort study

Purpose Thyroid hormones (THs) have multiple effects on lipid synthesis, mobilization, and degradation, suggesting that THs may affect the development of dyslipidemia. However, prospective studies on the association between serum THs levels and incident dyslipidemia in euthyroid subjects are limited. Therefore, we conducted a cohort study (similar to 5-year follow-up period, median: 3.0 years) to explore whether THs can affect incident dyslipidemia in a general euthyroid population aged 18 years old and over. Methods Dyslipidemia is characterized by elevated total cholesterol (TC), triglyceride (TG), or low-density lipoprotein cholesterol (LDL-C), or reduced high-density lipoprotein cholesterol (HDL-C). Serum free triiodothyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) were determined by chemiluminescence immunoassay. Multivariable Cox proportional hazards regression models were used to assess the association between baseline FT3, FT4, TSH, and the risk of various dyslipidemias. Results During follow-up period, the incidence of elevated TC, TG, LDL-C, and reduced HDL-C was 29.3%, 20.7%, 24.8%, and 19.5%, respectively. After adjustment for multiple confounders, we found that per unit increase in FT3 concentrations were associated with decreased incidence of elevated TC and LDL-C, and the hazard ratios (95% confidence interval) were 0.87 (0.79-0.97) (P < 0.01) and 0.897 (0.808-0.995) (P = 0.04), respectively. We also found a weak positive association between TSH and incidence of reduced HDL-C (P = 0.02). However, we found no association between FT4 and incident dyslipidemia. Conclusions Our results demonstrated that low FT3 was associated with high dyslipidemia risk, especially for elevated TC and LDL-C, and that TSH had a weak positive effect on incidence of reduced HDL-C.