Relationship of circulating C-reactive protein levels to thyroid status and cardiovascular risk in hyperlipidemic euthyroid subjects: low free thyroxine is associated with elevated hsCRP.

The mechanism(s) by which low circulating levels of thyroid hormones may lead to development of premature atherosclerosis remain to be established. These mechanisms include indirect effects of thyroid hormones on cardiovascular risk factors such as plasma lipoproteins, homocysteine and fibrinogen. High-sensitivity C-reactive protein (hsCRP) has been identified as an independent predictor of cardiovascular events. We presently investigated the relationship between hsCRP and free thyroxine (FT4) levels in a large population of euthyroid hyperlipidemic patients (n=429, mean age: 47.1 years, 28% of current smokers). None of these subjects presented a recent history of infection or inflammatory disease and those taking drugs known to influence thyroid or hsCRP were excluded. Serum FT4 levels were measured by radioimmunoassay and CRP, by a high-sensitivity immunoassay. In the population of non-smokers, plasma FT4 levels were negatively and significantly correlated with those of hsCRP (r=-0.13, P=0.02). Significant correlations between FT4 levels and age (r=-0.16, P=0.003), glycemia (r=-0.14, P=0.01), and fibrinogen (r=-0.18, P=0.001) were equally observed. Upon division of the population on the basis of FT4 tertiles, the mean level of hsCRP was significantly higher in non-smoker patients with the lowest FT4 tertile as compared to those displaying the highest FT4 level (3.04mg/l versus 1.77mg/l, respectively, P<0.05). No correlation between FT4 levels and CRP was found in smokers.In conclusion, we demonstrate that hsC-reactive protein is significantly negatively correlated with free thyroxine levels in non-smoker hyperlipidemic patients, suggesting that low thyroxine levels in euthyroid hyperlipidemic subjects constitute a new biomarker of elevated cardiovascular risk.