Context: Thyroid disease and the metabolic syndrome are both associated with cardiovascular disease.

Objective: To explore the hypothesis that thyroid function, in euthyroid subjects, is associated with components of the metabolic syndrome, including serum lipid concentrations and insulin resistance.

Methods: 2703 adult inhabitants of a middle-sized city in The Netherlands participated in this cross-sectional study. Subjects not being euthyroid were excluded, as were subjects taking thyroid medication, medication for diabetes and subjects of whom medication data were not available (n = 1122). HOMA-IR (mU*mmol/l²) was calculated as fasting insulin times fasting glucose (mmol/l) divided by 22.5. The metabolic syndrome was defined according to NCEP ATP III criteria.

Results: After adjustment for age and sex, FT4 was significantly associated with total cholesterol (standardized beta () = -0.059; P = 0.014), LDL-C ( = -0.068; P = 0.004), HDL-C ( = 0.100; P < 0.001) and triglycerides ( = -0.102; P < 0.001). Both FT4 and TSH were significantly associated with HOMA-IR ( = -0.133; P < 0.001 and = 0.055; P = 0.024 respectively). Median HOMA-IR increased from 1.42 in the highest tertile of FT4 to 1.66 in the lowest tertile of FT4. FT4 was significantly related to four out of five components of the metabolic syndrome (abdominal obesity, triglycerides, HDL-C and blood pressure), independent of insulin resistance.

Conclusion: We have demonstrated an association between FT4 levels within the normal reference range and lipids, in accordance with the earlier observed association between (sub)clinical hypothyroidism and hyperlipidemia. Moreover, low normal FT4 levels were significantly associated with increased insulin resistance. These findings are consistent with an increased cardiovascular risk in subjects with low normal thyroid function.