Are serum TSH levels associated with oxidized low density lipoprotein?

Introduction: Oxidized LDL cholesterol (oxLDL-C) plays a key role in the pathogenesis of atherosclerosis. Thus, it is important to investigate putative risk factors for increased oxLDL-C. Evidence suggests that, compared to euthyroid individuals, LDL-cholesterol (LDL-C) levels are lower in individuals with hyperthyroidism. In contrast it has been reported that oxidization of LDL-C is increased in hyperthyroidism. It has not been investigated whether subclinical thyroid dysfunction impacts oxLDL-C levels. We have analyzed the association between serum thyreotropin (TSH) levels and oxLDL-C in the population based cross-sectional „Study of Health in Pomerania“ (SHIP). Material and Methods: Data from 3519 individuals of SHIP was analyzed. Multivariable linear regression models were performed to assess the association between serum TSH levels and oxLDL-C levels. Fractional polynomials (FP) were applied to explore and graph nonlinear associations. In all multivariable analyses, TSH was transformed by a power transformation (PTSH) to reduce the effects of outliers on the FP. Results: The best FP to fit the association between PTSH and oxLDL-C levels was detected with a degree of 1 and a power of –2. PTSH^-2 was significantly associated with oxLDL-C levels (β=–0.23; 95%-confidence interval (CI)=–0.37, –0.09; p=0.001). Thus oxLDL levels increases in a curvilinear fashion with increasing serum TSH levels. Furthermore we detected a significant interaction term of PTSH^-2 and age. Subgroup analyses revealed a significant association only in the group of individuals >60 years. Additionally serum TSH levels were not associated with the ratio of oxLDL-C and LDL-C (β=–0.04; 95%-CI=–0.08, 0.01; p=0.087). Conclusion: We conclude that an association between serum TSH levels and oxLDL-C levels exists. Our study suggests that with respect to the association between serum TSH levels and oxLDL-C levels the formation of LDL-C is more important than its oxidation.