The metabolic syndrome and the thyroid in euthyroidism
A Roos1, SJL Bakker2, TP Links1 & BHR Wolffenbuttel1
Introduction: Thyroid disease is associated with cardiovascular disease and many cardiovascular risk factors cluster within the metabolic syndrome. It could therefore be hypothesised that thyroid function is associated with the metabolic syndrome.
Aim of the study: To investigate the relationship of thyroid function -in the euthyroid range- with serum lipid concentrations and with insulin resistance.
Methods: 2703 adult inhabitants of a middle-sized city in the Netherlands, participated in this cross-sectional study. Subjects on thyroid hormone replacement therapy or thyroid blocking agents, and subjects taking medication for dyslipidemia and/or diabetes were excluded. Since most patients with the metabolic syndrome will have normal thyroid function, we also excluded aneuthyroid subjects. The HOMA index for insulin resistance was calculated on the basis of fasting glucose and insulin levels.
Results: Significant, but weak positive correlations (all P<0.05) were found between TSH and HDL-C (r=0.06) and triglycerides (r=0.06). Significant negative correlations were found between: FT4 and total cholesterol (r=−0.06), LDL-C (r=−0.05), TG (r=−0.08) and Apo B (r=−0.05); FT3 and total cholesterol (r=−0.08), HDL-C (r=−0.05), LDL-C (r=−0.06) and Apo B (r=−0.06). FT4 (but not FT3) was negatively correlated with HOMA index (r=−0.14). The metabolic syndrome (according to NCEP ATP III criteria) was present in 21.0% of women and 16.6% of men and increased with age. FT4 in subjects with a metabolic syndrome was significantly lower than in subjects without a metabolic syndrome (12.6±1.7 vs 12.9±1.8 pmol/l; P=0.04)
Conclusion: We demonstrated an association between thyroid function and lipid levels in subjects classified as being euthyroid, in accordance with the earlier observed association between (sub)clinical hypothyroidism and hypercholesterolemia. Moreover, low normal FT4 levels were significantly associated with increased insulin resistance. This implicates that subjects with low-normal thyroid function already have increased cardiovascular risk.